The iPhone was launched last summer

One million iPhones, or 27% of those sold last year, have been adapted to work on other networks, said Bernstein Research analyst Toni Sacconaghi.

Apple recently admitted that the number of unlocked iPhones was "significant".

The iPhone, which was launched last summer, is tied to a single phone network in each country it is on sale.

While AT&T is Apple's chosen partner in the US, in the UK the iPhone is only officially available through O2, while in Germany it is tied to T-Mobile.

'Lost revenue'

Mr Sacconaghi said the high number of unlocked iPhones in the US would be a concern for Apple, as it receives a percentage of AT&T's contract fees from iPhone users.

He estimates that if Apple sells 10 million iPhones in the US by the end of 2008, but 30% of them have been unlocked to work with other phone networks, Apple will lose $500m (£251m) in revenues.

Apple and AT&T have declined to comment on the detail of the report.

Last month Vodafone failed in the German courts to get T-Mobile's exclusivity deal over the iPhone overturned.