Some early iPod touch units have "defective screens", according to Apple, which said the problem affects "a small number of units and is being remedied".

PC Advisor's sister title, Macworld US, said the display on one of the units it reviewed didn't match up to the image quality offered on the iPhone.

"The one I reviewed displayed video that was noticeably darker than that of my iPhone. For example, dark scenes in the classic Roman Polanski film, Chinatown, were really dark – lacking detail that the iPhone nicely detailed. The display also had a yellower cast than the iPhone. Dark images in photo albums similarly lacked detail," said Macworld’s Christopher Breen.

The iPod touch is due for release in the UK next week, although Apple is already taking pre-orders. See our iPod touch review here.