The EU's competition watchdog says it is examining whether five major publishing houses and technology company Apple have colluded to restrict competition in the market for e-books.
The European Commission's investigation focuses on potentially anti-competitive practices by publishers Hachette Livre, Harper Collins, Simon & Schuster, Penguin and Verlagsgruppe Georg von Holzbrinck.
The Commission said it would be looking at 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".
The announcement follows raids in the offices of several publishers earlier this year and a related probe launched by the UK's Office of Fair Trading.
Amelia Torres, the commission's competition spokeswoman, said the case would be treated as a matter of priority.
Apple is in a fierce battle over the growing e-reader market with US online retail giant Amazon, which launched a new version of its Kindle tablet in the US in September costing $199, half the price of the iPad.