Apple says iTunes sales aren't dropping

Research group Forrester said it was too soon to say whether its findings showed that buyers were "reaching their saturation level for digital music".

Apple said the report is "simply incorrect", but will not divulge iTunes financial figures.

Forrester said Apple users may be put off since iTunes songs can only be played on iPods or via iTunes itself.

This means that they cannot be used on rival digital library systems or music players.