It has issued a recall of two types of plug; the AC wall plug adapter shipped with Macs and some iOS devices between 2003 and 2010, and a three-pronged plug included in the World Travel Adapter kit.

The affected plugs were shipped in the UK, Singapore and Hong Kong.

Six incidents have been reported, Apple said.

In a statement, the firm said: "In very rare cases, affected Apple three-prong wall plug adapters designed primarily for use in the United Kingdom, Singapore and Hong Kong may break and create a risk of electrical shock if exposed metal parts are touched.

"Customer safety is always Apple's top priority and we have voluntarily decided to exchange affected wall plug adapters with a new adapter, free of charge."

It did not clarify how many people had received electric shocks.

Affected plugs are white, with no letters on the inside slot, unlike newer versions which are white with grey on the inside and have a dimple on the side to make them easier to unplug.

Apple USB power adapters are not affected.

The iPhone maker is in the middle of another plug recall, which affected two-pronged adapters for use in Continental Europe, Australia, New Zealand, Korea, Argentina and Brazil. These plugs had the same issue.