The iTunes music store, which has been running in the US since April 2003, opened for business in the UK, France and Germany in June last year. The three stores offer single downloads for a price of €0.99 in France and Germany, and £0.79 (€1.20) in the UK.
In October, a broader European version of the store was opened, extending the service to music fans in Austria, Belgium, Finland, Greece, Italy, Luxembourg, the Netherlands, Portugal and Spain at the price of €0.99 per song.
The issue came to the attention of the OFT in September when campaign group Which? (formerly known as the Consumers' Association) complained that the service charges UK customers 20% more than its French and German customers, and bars customers in the UK from logging on to the French and German sites to get a cheaper deal.
The OFT referred the matter to the European Commission in December.
Apple has made no comment on the investigation as yet.