Toyota Motor's Prius, the world's most popular hybrid car, is supposed to help save the earth from climate change but for now it has plunged the world's largest automaker deeper into trouble.

A rash of complaints about its braking system now raise the spectre of a safety recall of the firm's environmental flagship model, which would deal another blow to the embattled Japanese automaker.

Toyota has led the way in pushing forward fuel-efficient hybrid cars, which are powered by both petrol and electricity, amid growing public concern about greenhouse gas emissions blamed for global warming.

The Prius -- which runs on a world-beating 38.0 kilometres per litre, or 50 miles per gallon -- has created a global buzz and been Japan's top selling car since June ahead of its main rival, Honda's Insight.

Toyota had sold nearly 1.5 million Prius vehicles in 40 countries as of August 31 since the first version's launch in 1997, making it the world's most popular hybrid car.

Its success has been boosted by government subsidy schemes in the United States, Japan and elsewhere that aim to revive recession-battered auto sectors and lower automotive carbon dioxide emissions.

Toyota rolled out its third-generation Prius last May at a lower price tag starting from 2.05 million yen (then 21,580 dollars) in Japan, where the new model is manufactured.

It has a 1.8-litre petrol engine and Toyota's first electric-powered water pump, and, further boosting its green credentials, boasts solar panels in the roof to power the ventilation system.

Toyota said in December it plans to begin commercial sales of its next model, the first plug-in hybrid vehicle, in about two years.

Despite the hopes for and hype around Toyota's halo car, this week it added to the company's woes, after Toyota already had to recall some eight million cars over faults linked to the accelerator pedals.

Problems with the Prius focus on the brakes, and almost 180 drivers have complained of malfunctions in Japan and the United States, with drivers reporting a delay when they press the brake pedal, especially on bumpy or slippery roads.

Like the the Prius itself, the brakes are also a hybrid technology.

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

Some speculation over the braking problems has focused on the regenerative system, and whether a sharp jolt to the vehicle could inadvertently trick vehicle sensors and controls into releasing the brakes.

Apple co-founder Steve Wozniak, a Prius owner, this week said he believed Toyota's troubles may be linked to "some bad software" after his Prius sped up while in cruise-control.