US safety regulators have begun a formal probe into brake problems on the Toyota Prius, the world's best selling hybrid petrol-electric car, adding to the woes of the Japanese carmaker.

The National Highway Traffic Safety Administration said yesterday that it had received 124 complaints about braking problems on the 2010 model of the Prius, including four crashes. It announced the investigation after Toyota's disclosure earlier in the day that it had identified a design problem with the revamped Prius that can affect the performance of the car's brakes on icy or uneven roads.

Toyota, the world's biggest carmaker, has recalled more than 8m vehicles since November to fix accelerator pedals and out-of-position floormats, which have been linked to crashes. Yesterday, it put the financial cost of those recalls and a related sales and production freeze at up to Y180bn ($1.98bn).