Felipe Massa says he is "ready to follow a different path" after announcing he will retire from Formula 1 at the end of the 2017 season.

Massa originally quit F1 at the end of 2016, but was enticed back by Williams to replace Valtteri Bottas, who left for Mercedes ahead of the current campaign following Nico Rosberg's surprise retirement.

Massa admits he was keen to stay on in F1 for a 16th season, as he feels competitive enough to compete, but has "no regrets".

"Part of me wanted to carry on because I think I'm still competitive enough to race a top-line Formula 1 car, and also because I feel very good at Williams," he said in his column for Autosport's sister title Motorsport.com.

"But every story has to come to an end at some point, and on my part I have no regrets.

"Exactly a year ago I lived the same situation, and then suddenly the team asked me to come back.

"For me, it was a real privilege to do so and something I appreciated a lot, and a year later I am ready to follow a different path - happy with what I have done and what we have achieved this season."

Massa plans to continue racing, with Formula E on the agenda in the future, but has yet to decide what his programme will be next year.

"There are those who will be curious to know where I will be racing next year, but it is too early to declare now," said Massa, who has scored 11 wins, 41 podiums, 16 pole positions in F1 and finished runner up to Lewis Hamilton in the 2008 drivers' championship.

"For now my focus is on finishing the season. I have two more grands prix to do, which will be important for me and the team, and then you will see.

"I will consider all the possibilities that I have been offered, and then decide without hesitation nor with the stress of being on track at all costs. I like to race, I feel competitive, and that's what I want to do.

"I would like to accept a new challenge, as long as it is a professional role and with good prospects to deliver strong results.

"While I enjoy racing, I want to keep doing it - and keep getting the pleasure that I have always had throughout my career."

McLaren says it is "giggling" with excitement about what it can achieve in Formula 1 next year, on the back of its engine switch and progress with its chassis.

After securing a Renault customer supply to replace its Honda engine deal, the Woking-based team has been buoyed by the step forward in pace a new front wing concept has brought.

The development has been so good that Fernando Alonso reckons that McLaren now has the 'best' car in qualifying.

McLaren executive director Zak Brown said the step forward the team has made in recent races is hugely encouraging for next year, and is raising excitement levels about what can be achieved in 2018.

"Almost everything that we have developed over the year in the factory corresponds to the car, so we have a high strike rate of success," Brown told Autosport.

"Sometimes it can work in the factory and not on the car, but the chassis has been getting better and better and better.

"Our GPS says we are right near Red Bull: maybe a little better at some tracks and a little worse at other circuits.

"But they have won a couple of races, so we are cautiously optimistic [for 2018]. We almost have a few giggles getting ready for next year. We are excited."

Although there is no dramatic change to the F1 car regulations for next year, some tweaks to the rules - including the arrival of the halo, plus the banning of monkey-seats and T-wings - will have an impact on aero designs.

That is why McLaren racing director Eric Boullier is slightly cautious about having total confidence in his team fighting right at the front in 2018.

"You can never ever have faith of this," he said. "If it was the same regulations 100% then I would say yes.

"But obviously there will be some changes next year: no T-wing, no monkey seat, a lot of cleaning up at the back of the car which makes it a little bit harder - plus the halo, which obviously is a disaster in terms of aero. So there is a lot of work to do around this.

"But I trust the team we have, and what they achieved in the last three years.

"There is no reason why they cannot keep doing it, building up and catching up."

When asked if he was at least confident of McLaren joining the fight among Mercedes, Ferrari and Red Bull, Boullier said: "Yes. 100%."

Haas Formula 1 team principal Gunther Steiner says the 2021 engine parameters announced by F1 and the FIA are "a good concept", but acknowledges they will need substantial refinement.

Manufacturers Ferrari, Mercedes and Renault all expressed their disappointment with the plans, which focus on less technology and more standardisation of parts.

Steiner, who is keen to see costs reduced and the playing field levelled, is the first team boss to offer a positive response.

"They've tried to achieve the things they set out with more noise, more equality and lower costs," he said.

"That is the aim of it. I think they've thrown out a good concept to start off with.

"Now the details can be worked out by the technical people.

"The concept is out there, and I don't think the concept will be changed.

"But now they need to work on the detail of the concept to achieve the goals they've set themselves with more noise, more equality and lower costs for the customer teams.

"Hopefully, they can achieve it."

While reducing costs is a key issue for teams such as Haas, Steiner accepted that the engine has to tick other boxes and be acceptable to fans.

"I wouldn't say [cost] is the most important. It's as important as the other ones because even if it's cheap, if people don't like the engine, why would we do it?" he said.

"There needs to be a compromise between what the fans like, which is noise, and new manufacturers coming in, which is what people want.

"The cost element is also important for new people, for current people, and for the teams which don't make their own engines, like us.

"It's part of what we need to do to change F1 to be better."

Steiner stressed that getting the sound right has to be a key goal.

"The noise - if you don't have it, you think it's not important because it's just noise. If you hear a V10 or a V12 going by, when you see these historic cars, it sounds beautiful," he said.

"I think a lot of people like the noise. I don't think we'll get to that noise of a V10 or a V12, because you have a turbo on it.

"You can improve, and the aim is with making the rev limit 3000 rpm higher than the current engine, it can be achieved to be noisier.

"I think it will never be like it was. A little bit noisier is good because I think it's great for the fans to hear a car coming from far away.

"I think it will take a year to finalise the regulations. I think that's the aim, to have the final version of the technical regulations by the end of 2018.

"Then it's a two-year development process. I think already the manufacturers will start now to develop, because you cannot be behind."

Independent engine companies Ilmor and Cosworth have backed Formula 1's proposed new rules as a potential entry route for new competitors.

The pair say it is almost impossible for a new company to compete with the current engine suppliers under the existing rules.

Cosworth managing director Bruce Wood said: "The current regulation is beyond any new entrant, technically and for the commercial investment it requires.

"The new proposal makes it possible for an independent or existing car company."

Cosworth was the last independent engine manufacturer to take part in F1, before pulling out after 2013.

Steve Miller, managing director of Ilmor which ran the Mercedes F1 engine programme until the early 2000s, said the new rules proposed last week by governing body the FIA and the F1 Group from 2021 "open the scope to a much broader range of manufacturers" by making it cheaper to design and build an F1 engine.

However, both companies say they would need investment from an external backer before they could contemplate making an F1 engine, even under the proposed new rules.

The proposal by FIA and F1 is to retain the current engine architecture of a 1.6-litre V6 turbo hybrid but simplify it by removing the most complex part of the hybrid system - the so-called MGU-H, which recovers energy from the turbo - and impose restrictions on a number of parts.

Three of F1's current engine manufacturers, Mercedes, Ferrari and Renault, have expressed misgivings about aspects of the plan.

Mercedes and Renault say it will increase their costs by forcing them to effectively design an all-new engine, while Ferrari say it diminishes "power-train uniqueness" and have threatened to quit F1 if the new rules are not to their liking.

Ilmor and Cosworth, in exclusive interviews with BBC Sport, say:

• Changing and simplifying the engine rules is effectively the only way F1 could attract new engine companies
• Even under the new rules proposal, small companies such as them would still need a partnership with a car company to design and build an F1 engine
• Removing the MGU-H is key if F1 wants new entrants
• It will always be difficult for smaller companies to compete on a level playing field with giants such as Mercedes and Ferrari, whatever the rules

The proposed engine rules will be discussed at a meeting of F1's strategy group of leading figures on Tuesday, along with other aspects of the vision for the future of the sport by new owners Liberty Media, which bought the F1 Group earlier this year.

F1 and the FIA are concerned that the engine rules are over-complex and require too great a financial investment to attract new companies.

They fear this makes F1 too dependent on the existing car manufacturers and vulnerable if they decide to quit.

There are also concerns that the engines lack visceral appeal to spectators. Although they have set new benchmarks for efficiency with their revolutionary technology, their sound is widely considered to be unattractive.

They want a new engine concept that is cheaper, noisier, simpler, that enables new companies to enter F1 and is more attractive to fans.

Miller says: "It's inherently the right thing to do to get away from an over-complicated product that doesn't sound good and is not really turning on the fans.

"If they come out of this with a measured view of what's required to get more manufacturers on the grid, then it is easily achievable from the platform they are working from and I think they are headed in the right direction."

Wood said: "We think the (proposed) rules are something that open it up to a much wider group of people technically and to a much smaller amount of money, which can only interest more people.

"The key thing is that anybody new coming in would have to spend a lot, lot less.

"Certainly no independent could contemplate it under the current rules and any car company or anyone else entering now knows it is a many hundreds of millions bill and in the current world I'm not sure anybody can tolerate that."

Wood says he can "see why the proposal is controversial" but adds: "It is impossible to say the status quo as it is now can be maintained for the long term and if one of the current players should drop out, I don't think they are going to be replaced by anybody so I think they have taken a long-term view and said: 'It's probably not going to be popular but it does protect the sport longer term.'"Why are the new rules controversial?The existing manufacturers have collectively ploughed hundreds of millions into the design of the existing F1 engines and are reluctant to see that wasted.

One particularly controversial aspect of the new proposal is the decision to ditch the MGU-H, which provides 60% of the total hybrid energy of the current engines, the best of which have around 1,000bhp at full power.

Wood says the MGU-H is "a huge part" of why it would be so difficult for a new company to enter F1 under the current rules.

"It's hugely expensive, hugely complex and you're always going to be many years behind starting from ground zero with heat energy recovery," he added.

But Renault Sport managing director Cyril Abiteboul told BBC Sport: "We actually believe that the MGU-H is a fantastic device in order to have sustainable power around the lap and during the whole race. So that is the sort of disconnect we may be finding ourselves in right now.

"My problem with the removal of the MGU-H is that as soon as you do that it is a new engine.

"It fundamentally changes the way the energy is managed within the engine, the way the turbo is working and so on and so forth. It is a new combustion concept, a new way to manage turbo lag and efficiency, so it is new design of turbo and so on - a new engine."

Abiteboul said he would prefer to try to resolve the aims of the new rules by tweaking the existing engines rather than introducing a new one.

Mercedes F1 boss Toto Wolff said: "What we need to look at is costs, certainly development costs for the OEMs (car manufacturers). It is an area we need to tackle.

"We also understand that the quality of sound is something we need to improve. But we must avoid developing a new engine concept that will trigger immense costs."

But will new companies come in?

Just because a new engine concept makes it easier for new entrants in F1, does not necessarily mean any will take the plunge.

Miller said "the prospect of getting external backing (for an Ilmor engine) is very, very unlikely" but added that it was "likely" smaller car companies than the giants in F1 "are going to look at it as a viable prospect and they may well decide the most viable way forward is to use sub-contractors such as ourselves and Cosworth".

Wood said that smaller manufacturers would always be at a disadvantage compared to the likes of Mercedes but that the proposed new rules would enhance their ability to compete.

"There has been a lot of talk of levelling the playing field," Wood said. "I don't think it does level the playing field. Mercedes/Ferrari/Renault/Honda have got hundreds if not thousands of hugely intelligent people working on this. But what it does mean is a law of diminishing returns.

"Whereas at the moment there is a disparity of 5-10% up and down the grid depending on who's got it right and who's got it not quite so right, with a regulation of this sort, you reduce that disparity to maybe 1%."

However, although the VW Group, Aston Martin, Italy's Magneti Marelli, Austria's AVL, Ilmor and Cosworth have all attended meetings about F1's future engine rules, none have so far committed to enter.

And Mercedes and Renault have told the FIA and F1 that they risk upsetting the current suppliers with no guarantee they can be replaced.

Abiteboul said: "The first objective of any new rule should be hopefully to keep the existing manufacturers.

"I am absolutely not putting a threat. But what I am saying is that F1 is not doing that bad right now given the overall economic situation of the world.

"You have a number of teams, diversity of teams, four engine manufacturers, three engine manufacturers were competitive in the last weekend. Maybe not equally reliable but at least equally competitive and that's not that bad.

"So we need to be a little bit careful not to try to fix something that is not broken. F1 is not broken. It might be good to have more manufacturers but we should not jeopardise all of F1 just to try to attract someone else."

Red Bull boss Christian Horner believes that bringing forward the release of his team's 2018 Formula 1 car by just five days will be enough to help it attack from the first race next year.

The Milton Keynes-based team has again made great progress through this season, having turned its RB13 into a race winner and outscored Ferrari since the summer break.

But the team paid the price for a sluggish start to the year, where it only realised too late that there were wind tunnel correlation issues with its car that hampered its form early on.

In a bid to ensure that it is not so exposed to discovering problems too late, Red Bull is to next year shy away from its traditional policy of holding back the release of its car until the last possible moment so it can maximise development time.

Instead, it will bring forward the car finish date to five days earlier than it would have done before, something which Horner thinks will help it hit the ground running right from the off.

"We are focusing on a slightly earlier release target, which only involves about five days, but the design is so concertinaed and production schedules so tight, that those five days are actually pretty valuable in terms of being on the front foot rather than the back foot," he said.

"Our intention is to try to turn up at the first test in a position to knock about 100 laps out."

Red Bull Racing has won two out of the last four races, thanks to Max Verstappen's success in Malaysia and Mexico, and was even able to battle for pole position in qualifying last weekend.Horner believes that the fact there is no huge regulation change next year means there is some confidence within the organisation that it can carry its strong form forward.

"We've got pretty much regulation stability, so the lessons we take out of RB13 will go into RB14," he said.

"Obviously we are hopeful that on the engine side that performance and reliability improve over the winter, and they are fundamental aspects for us.

"We have demonstrated that we have got a really competitive chassis. I think since Hungary we've been very, very strong.

"So as I say, if we can take these learnings into next year's car then hopefully we can start on a stronger footing than the second or so we were off in Melbourne at the beginning of the year."

The Mercedes Formula 1 team will use the final two grands prix of 2017 to trial "new and interesting" concepts for next season.

Mercedes has wrapped up its fourth consecutive constructors' and drivers' championship double early, despite an intense battle with Ferrari and Sebastian Vettel for the respective honours.

This weekend's Brazilian Grand Prix and the season-closing Abu Dhabi event across November 24-26 give Mercedes "the first two test opportunities ahead of the new season, trialling new and interesting concepts".

In its preview for the Interlagos race, Mercedes pledges to "become a little bolder", test new parts and carry out "experiments" that would be too risky if it was still fighting for titles.

Mercedes motorsport boss Toto Wolff said: "It might be tempting to think that, with both championships now secure, the pressure is off for the two remaining races of 2017.

"That couldn't be further from the truth.

"Inside the team, we are looking at the next two race weekends as the first two grands prix of 2018. We have two races that we are determined to win in order to take that positive momentum into the winter.

"There will be no backing off just because the championship business is now done."

Lewis Hamilton clinched his third title in four years with Mercedes in the Mexican Grand Prix two weeks ago.

His new-for-2017 teammate Valtteri Bottas trails Vettel by 15 points in the fight to finish second in the drivers' championship.

Wolff said both drivers should expect to win in Brazil but predicts Ferrari and Red Bull, which claimed pole and the race victory respectively in Mexico, will be legitimate contenders.

He added: "Lewis is operating at the peak of his powers right now, and will be determined to add another victory to last year's success; Valtteri made a promising step forward in Mexico and will aim to build from this at Interlagos.

"As recent rounds have shown, winning grands prix is never easy. Red Bullhave taken two victories in the past four races and, although we have claimed the other two, Ferrari remain formidable opponents.

"With the championship now settled, the battle for 2018 has already begun."

World champion Lewis Hamilton says the move to limit drivers to just three engines next season "sucks".

Drivers are limited to four engines this season before incurring grid penalties, and the rules are cutting that back in 2018 to reduce costs.

Hamilton said reducing the number of engines would mean drivers were able to push hard in races less often.

"I don't like the idea of going to three. That sucks. Sprinting is what we are missing in F1," Hamilton said.

He said increasing the need to manage engines in races to extend their life combined with the weight of the cars - which is going up next year because of the introduction of the 'halo' driver cockpit head protection system - made racing more unappealing.

"The car is going to be a bus next year, it is going to be so heavy, like a Nascar (stock car)," Hamilton said after the Brazilian Grand Prix, in which he was able to push his engine hard because a new one was fitted to aid his fight back from a pit-lane start.

"The braking distances get longer, the brakes are always on fire, on the limit.

"I know it sounds negative but as a racer we want fast, nimble cars where we can attack always every single lap.

"Unfortunately that is not what we generally have. I had that today but I was coming from a different place.

"If you look at the front guys, they were managing and that is what we are normally doing.

"I don't think that is too exciting for people to watch. If you look at the most exciting races - particularly when it rains - we don't have those limitations.

"I'm not sure cutting down engines is helping it in that direction."

Hamilton is not alone in questioning the move to three engines.

Red Bull team boss Christian Horner said he believed five engines per driver year was the ideal number. But attempts to stop the move to three next year have so far failed.

There is also a widespread concern in F1 that the grid penalties that are handed out when a team uses more than the permitted number of engine parts are making F1 a farce.

The McLaren-Honda drivers alone have been hit with a total of nearly 400 grid penalty places in the 19 races so far this season and team insiders say they expect to have more at the final race in Abu Dhabi on 24-26 November.