WASHINGTON -- Under a new federal rule, an automaker must notify its dealers within three days if the company plans to recall a vehicle with a defect that poses "an immediate and substantial threat" to safety.

Such a notice effectively stops sales of the vehicles and reduces the number that consumers otherwise would have to return for repairs.

Manufacturers can continue to notify dealers of recalls of other vehicles within "a reasonable time."

Regulators accepted industry arguments that stopping sales of cars and trucks with less serious defects can create a financial hardship, especially for dealers with large vehicle inventories.

The National Highway Traffic Safety Administration adopted the three-day rule even though it says manufacturers have been highly responsible about promptly and voluntarily stopping the sales of vehicles that are thought to pose a serious threat.

But the adoption ends a debate that has been going on, mostly behind the scenes, for more than a decade. And it represents a small concession to safety and consumer groups that long have argued for a definite timetable for notifying dealers of all safety recalls.

Clarence Ditlow, director of the Center for Auto Safety, one of the groups that had petitioned for quick notices to dealers of all recalls, declared a partial victory.

"These days, half a loaf is not bad," Ditlow says.

The debate goes back to 1993. NHTSA first proposed notifying dealers of any recall within five days of finding a defect. Automakers and dealers objected, and action on that provision was postponed.

The agency proposed another approach in 1999. Under that plan, automakers would have submitted a notification schedule to NHTSA, but the agency could have ordered changes if it saw a need.

A version of that proposal was adopted in mid-2004, but Center for Auto Safety and the consumer group Public Citizen petitioned for reconsideration.

NHTSA's reconsideration led to the adoption this month of the three-day rule. It applies to electronic communications with dealers. Automakers can take up to five days if they use regular mail.

The agency, in its public notice of the change, emphasized that it still believes automakers have been responsible about prompt notices to dealers of serious defects. It cited action in May by Mitsubishi Motors North America Inc. to stop sales of the 2006 Eclipse as soon as a major brake problem was found.