The Japanese carmaker has received several complaints about braking in the Prius in both Japan and North America since the end of last year, Toyota spokeswoman Ririko Takeuchi said.

She did not say when the directive was issued or how Toyota would respond. She also declined to give a figure for the number of complaints but said most were filed since the end of last year.

News of the fresh investigation comes as the world's largest automaker struggles to rein in a recall crisis aimed at fixing incidents of unintended acceleration in some of its vehicles. Since last fall, Toyota has recalled 8.1 million vehicles worldwide--more cars than it sold last year.

Problems with the latest generation of the Prius, which went on sale in the United States, Europe and Japan last year, would be another blow. The green halo surrounding the popular hybrid is largely credited for Toyota's image as a leader in environmentally friendly technology.

“All the complaints were nearly the same,” Takeuchi said. “They are usually about driving in bumpy or icy conditions and when you push the brake, there is an unusual feel.”

Braking issues detailed

A computerized search for the 2010 Prius on the online data base at the National Highway Traffic Safety Administration generated at least 102 complaints against the car.

The U.S. complaints accounted for at least two injuries. Drivers typically complain about losing braking ability--often while driving over bumpy roads--or feeling the brakes give way.

A report filed Feb. 1 reads: “While braking, if I hit potholes, the car speeds up from loss of braking action, even thou (sic) I am still pushing on the brake pedal.”

Another complaint filed in September 2009 said the driver injured her neck when the brakes failed on her Prius and she collided with another vehicle.

The Japanese government has received 14 reports of braking problems in the Prius, including one that caused an accident with two injuries in July, media reports here said.

A blow to the hybrid leader

In Japan, the Prius is the best-selling car. It is the best-selling gasoline-electric hybrid model in North America and Europe.

Officials at Japan's Ministry of Transportation were not immediately available for comment.

In Japan, 13 of the 14 complaints came in December and January, reports said. In the July incident, a Prius crashed into another car at an intersection, injuring two people.