More than a quarter of iPhones sold in the US have been "unlocked" to work on network providers other than Apple's exclusive partner AT&T, a study says.

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.