An exemption to the Digital Millennium Copyright Act (DMCA) in the US means that it may be legal to unlock an iPhone and use it on networks other than AT&T's.

A report in Business Week suggests that the exemption, which is in place for three years, covers computer programs that 'enable wireless telephone handsets to connect to a wireless telephone communication network, when circumvention is accomplished for the sole purpose of lawfully connecting to a wireless telephone communication network'.

Individual users could therefore legally unlock an iPhone until the exemption runs out in November 2009.

Lawyers for Apple and AT &T have tried to deter hackers from unlocking iPhones in order to protect the monthly service charges they receive.

The two firms are expected to claim that a statement within the DMCA protects the iPhone from being unlocked because it is a copyrighted work.

The DMCA bill reads: 'No person shall circumvent a technological measure that effectively controls access to a work protected under this title.'

However, users who unlock their iPhone will not escape AT&T network charges completely if they move to another carrier.

The Apple phone, which went on sale on 29 June, was sold with a two-year contract and includes an early termination fee of $175.