Apple has launched an exchange programme for some of its iPhone chargers which the firm says may "overheat and pose a safety risk".

The technology giant has warned that in "rare cases" the Apple 5W European USB power adapter sold between October 2009 and September 2012 may not be safe.

In a statement on its website, the company said it decided to launch the exchange programme as a precaution because safety is their "top priority".

Apple said they will exchange every affected power adapter for a new one, free of charge.

"We encourage customers to exchange any affected adapter for a new one as soon as possible using the exchange process below," the company said on its website.

The European USB power adapter came with iPhone 3GS, iPhone 4 and iPhone 4S models shipped from October 2009 to September 2012. It was also sold as a standalone accessory.

The charger was sold in 37 countries.

Apple has urged customers to visit the company's website for full details of the exchange programme.