Toyota issued a recall today for all 1.9 million of its third-generation Prius cars sold worldwide, due to a software programming problem which could cause the car to brake unexpectedly.

About half of the recalls are in Japan and 713,000 in North America, a Toyota spokeswoman said. No accidents have been reported related to the defect, although there have been 400 reports of the fault.

Toyota said the problem was in the software used to control the boost converter in a module that is part of the hybrid system.

Toyota has identified that there may be an issue concerning the software used to control the boost converter in the intelligent power module, part of the hybrid system’s inverter assembly. The boost converter is required when driving with a high system load, for example when accelerating hard from standstill.

The software setting could lead to higher thermal stress occurring in certain insulated-gate bipolar transistors in the boost converter which may lead to them deforming or being damaged.

Should this happen, warning lights may be illuminated and the car is likely to switch to “failsafe” operation – it can still be driven, but with reduced power. In limited cases the hybrid system may shut down, causing the vehicle to stop. The driver will not experience any change in the vehicle’s behaviour or performance prior to the problem occurring.

The software update will take about 40 minutes to complete; the work will be carried out free of charge.

Toyota can confirm that this issue will not occur in other Toyota hybrid vehicles, as these use different transistor structures and control logic.

Toyota, the world’s biggest car manufacturer, did not disclose cost estimates for recalls.

In 2010 Toyota recalled more than 10 million vehicles over unintented acceleration issues. It is reported to have reached a $1.1 billion settlement to compensate Toyota and Lexus owners for the loss in resale value of their vehicles, despite many of the complaints being found to be the result of driver error, not technical faults.

> Toyota recall helpline: 0800 1388744