The European Commission has opened formal antitrust proceedings to investigate whether five international publishing houses have engaged in anti-competitive practices affecting the sale of e-books in Europe.

Apple's role in the sale of e-books will also be investigated alongside the probe into Hachette, Harper Collins, Simon & Schuster, Penguin and Verlagsgruppe Georg von Holzbrinck.

"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 European Economic Area," the EC said in a statement released this morning.

"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 e-books."

The EC said it "had informed the companies and the competition authorities of the Member States that it has opened proceedings in this case". It is not known how long the investigation will take but the EC said it would depend on the complexity of each case, the extent to which the undertakings concerned cooperate with the Commission and the exercise of the rights of defence.

The opening of the formal proceedings comes after a joint investigation between the EC and the Office of Fair Trading (OFT) into the sale of e-books. The OFT had closed its investigation "on grounds of administrative priority" but will continue to co-operate with the formal proceedings.