The modifications address concerns raised by GM and the National Highway Traffic Safety Administration (NHTSA) last November, after two crash-tested Volts caught fire at its storage facility.

As a result of its investigation, GM has developed a modified protective structure around the battery pack and added a system to monitor levels and prevent overfill of its coolant system. The manufacturer said it had conducted four crash tests in December, and seen no intrusion into the battery compartment or coolant leak as a result.

Karl-Friedrich Stracke, Opel chief executive officer, explained: 'The Ampera has always been safe to drive. Now, our customers will have the additional peace of mind that the Ampera is equally as safe in the days and weeks following a severe crash.'

All Volt and Ampera models built after the Christmas break will be fitted with the new modifications at the factory. Existing Volt customers in America will be notified when these can be retro-fitted to their vehicles.

In Europe, the Ampera is due to be delivered at the end of February. Cars manufactured before the Christmas break will be upgraded before delivery.