DETROIT: Toyota is facing a potential safety issue with its highest profile vehicle, the Prius, the latest in a plague of quality problems that forced it to recall four million vehicles in 2009. A growing number of owners allege that the brakes on the third-generation, 2010 Toyota Prius can malfunction unexpectedly, with at least 20 complaints filed so far with the National Highway Traffic Safety Administration.

The Japanese automaker said it has launched its own investigation.

Robert Becker, 39, is one of those filing among at least 20 who have already submitted their concerns to the National Highway Traffic Safety Administration's Office of Defects Investigation.

He says he was heading to work on the west side of Manhattan, coming up to an intersection and squeezing the brakes of his 2010 Prius to slow down.

But when the car hit a pothole, Becker suddenly had the "sensation of losing control," as the brakes released, forcing him to slam down on the pedal.

"It scared the hell out of me. I wasn't sure I could stop in time," he said, adding the problem has repeated itself a number of times since then.

Becker is not alone, as NHTSA's defects office reveals.

One complaint on file quotes an owner: "Initially, I convinced myself I must have been letting up on the brake when I hit the bump, but when this same thing happened three days ago on slippery, icy roads, I knew for 100 percent certain I had not let up on the brake."

The Prius brake problem has become a hot topic on numerous websites, but federal investigators are so far declining to comment, although the file is open to the public.

Toyota admits it is aware of what a spokesman called "the behavior people are reporting."

"We're investigating those complaints as quickly as possible," spokesman Mike Michels added.

Exactly what is happening is unclear. Like the vehicle's gasoline-electric powertrain, the brakes are also a hybrid technology.

During light to moderate braking, the car is slowed by a regenerative system that turns the vehicle's kinetic energy into electricity, which is then stored in a battery. For more aggressive stops, the Prius also has a conventional hydraulic brake system.

Some speculation focuses on the regenerative system, and whether a sharp jolt to the vehicle could inadvertently trick vehicle sensors and controls into releasing the brakes.

But Michels cautioned Toyota will have to look at a variety of things. "Rather than throwing out theories, the important thing is to do a scientific analysis. When we have an answer, we will provide it to owners as soon as possible," he said.

The Prius problem is one of the last things Toyota needs right now.

Just 12 months ago the automaker was basking in headlines reporting that it had finally beaten arch-rival General Motors to become the world's best-selling automaker.

But by March, things didn't look so good. Battered by the recession in the US, its key market, the maker reported its first annual loss in a half century.

In August, a California Highway Patrol officer and three members of his family were killed in a fiery crash when their Lexus went out of control.

Two months later, Toyota announced it would recall 3.8 million vehicles, blaming the problem on floor mats that could come loose and jam the accelerator pedal.

Another recall impacted 110,000 Tundra pickups which, according to NHTSA, are prone to "excessive corrosion" so severe their brakes could fail.

In all, Toyota will have recalled around four million cars, trucks and crossovers, in the United States, during 2009. That's about four times more than in previous years.

And it means Toyota will have recalled more vehicles than any other auto manufacturer for the first time ever.

It is looking like 2010 could also be a difficult year. Another probe was recently opened by the government into complaints that 2006 versions of Toyota's Corolla and Matrix models may be prone to unexpected stalling, sometimes at highway speeds.

Although Prius sales will only account for about 100,000 units this year, less than seven percent of Toyota's US total, it has been positioned as the company's halo vehicle due to its environmentally-friendly hybrid technology.

But analyst Stephanie Brinley, of AutoPacific, Inc., warned that if it is suddenly seen as dangerous, the damage to Toyota could be huge.

Early customers may be more willing to accept problems with new technology, she added, but for "late adopters, who care more about mileage and less about technology, this could be a red flag."