The EU is to investigate Apple and five publishers over the price of eBooks.

Hachette Livre, Penguin, Harper Collins, Simon & Schuster and Verlagsgruppe Georg von Holzbrinck are also being investigated by the European Commission, which is concerned the firms may have "engaged in anti-competitive practices" regarding the price of eBooks, "possibly with the help of Apple", which breaks EU antitrust regulations.

Apple iPhone and iPad users can purchase eBooks for their devices from Apple's iBook store.

"The Commission will in particular investigate whether these publishing groups and Apple have engaged in illegal agreements or practices that would have the object or the effect of restricting competition in the EU or in the EEA," the EU said.

"The Commission is also examining the character and terms of the agency agreements entered into by the above named five publishers and retailers for the sale of eBooks."

The Commission did not say how long the investigation will take but said: "the opening of proceedings means that the Commission will treat the case as a matter of priority."

The UK's Office of Fair Trading (OFT) has already carried out a similar investigation, which began in February this year. However, it has since been closed "on grounds of administrative priority".

The EU said the OFT "has made a substantial contribution to the eBooks investigation and will continue to co-operate closely with the Commission going forward".

Apple has yet to comment on the issue.