A quarter of iPhone users are not signing up to the contracts demanded by Apple, according to new research.

A million iPhones, or 27 per cent of the total sold, have not been registered with one of Apple's network providers, namely O2 in the UK, T-Mobile in Germany and AT&T in the US, according to Bernstein Research.

The analyst firm estimates that Apple has lost £251m as a result, as the company takes a cut of all data revenues carried on the network.

Gene Munster, a senior research analyst at investment bank Piper Jaffray, claimed that the rogue iPhones are being bought by organised gangs which unlock the handsets and sell them overseas predominantly in Asia.

The analyst monitored purchases in Apple stores in New York, San Francisco and Minneapolis, and claimed that over 40 per cent of users were buying more than one phone at a time.

"The majority of the people who were buying more than one phone were Asian, and they were bringing small buses of people who all buy more than one phone," he told The New York Times.

Munster explained that the phones were probably being sold for use in Asia because the value of the dollar made the cost of the phone much less in the US.

Apple may not be too concerned about the grey market in iPhones, however, since it already makes significant profits from the hardware itself.