Apple has admitted some models of its iMacs are prone to freezing, and has promised a fix as soon as possible.

"A small number of iMac users have made us aware that a recent software update issued by Apple can sometimes cause their iMacs to freeze, requiring them to restart their iMac," an Apple spokesperson told PC Advisor's sister title Macworld US.

According to users the computer becomes unusable, requiring a hard reset in order to recover. Apple doesn't know the exact cause of the problem, but it is investigating.

"We are tracking down the root cause of this bug, and will issue a software update which corrects it as soon as we can - most likely later this month," the company spokesperson said. "We apologise for the inconvenience."