The US Department of Justice has warned Apple and five US book publishers that it is planning to file lawsuits against them for allegedly colluding to fix the prices of ebooks, according to a media report.

The DOJ is preparing lawsuits against Apple, Simon & Schuster, Hachette Book Group, Penguin Group, Macmillan, and HarperCollins Publishers, according to a report in the Wall Street Journal, quoting anonymous sources.

This follows investigations by the European Commission in December 2011 into the same companies for the same reason, but the inquiry has not yet been completed. 

A DOJ official told US lawmakers in December that the agency was investigating ebook pricing. That announcement followed one by the European Union, which said it was investigating Apple and the same five publishers.

A DOJ spokeswoman declined to comment on the new report, but said the ebook investigation is still open.

Apple didn't immediately respond to a request for comments.

Critics accuse Apple and the five publishers of setting prices in response to's attempt to introduce a US$9.99 (£6.32) rate for most ebooks.