Hacking the Apple iPhone so it can be used on any mobile network is proving more difficult than expected. The situation is causing red faces among hackers who said they'd crack the iPhone in a week .

Hackers had hoped that by modifying the bootloader - the program that runs before Mac OS X has loaded on the device - the Apple iPhone could be unlocked. However it turns out that the bootloader code must be signed using an 128-bit private encryption key provided by Apple . That's according to an update posted on the #iphone internet relay channel (IRC).

#iphone hackers are now trying to create an assembler utility for the processor inside the Apple iPhone. This should enable them to write a program that can unlock the handset, according to Macworld UK .

Unlocking the Apple iPhone would enable it to be used with other mobile operators than AT&T, including operators in the UK and other countries where the device has yet to go on sale. The Apple iPhone's activation code was bypassed within days of it going on sale on Friday 29 June.