But on Wednesday, Japan’s Transport Ministry said it had told Toyota to investigate 14 reports of momentary brake failure since the car went on sale in May.

Drivers complained that the brakes on the newest Prius momentarily stopped working at low speeds, especially on slippery surfaces, said Kenji Sato, a Transport Ministry official. In one incident in July 2009, a Prius crashed head-on into another vehicle at an intersection, hurting two people in that car.

In the United States, the National Highway Traffic Safety Administration has logged at least 136 complaints about the brakes on the 2010 Prius. Many are from drivers who say the vehicle surged forward or temporarily lost braking after driving over a pothole or other uneven surface, and many say it is a recurring problem.

Four cases involved a crash, and two of those resulted in injuries, according to the safety agency’s complaint database.

Ms. Takeuchi stressed that the newest Prius model was fitted with an overhauled regenerative brake system different from the ones used in previous models, which had not caused any complaints.

“We are examining any complaints that we hear through our dealers, one by one,” said Shinichi Sasaki, an executive at Toyota in charge of quality. “Only after that can we get back to our customers on possible improvement.”

Experts say a possible problem with the regenerative brakes in the Prius raises a conundrum not just for Toyota but for the entire industry. It is also bad news for environmental advocates who considered the technology one way to help improve fuel economy and reduce the contribution of automobiles to global warming.