Apple is guilty of leading a conspiracy to fix the price of e-books with publishers, a US judge has ruled.
Five publishers, including Penguin, who were initially listed as defendants alongside Apple all settled before the case.
"Without Apple's orchestration of this conspiracy, it would not have succeeded as it did in the spring of 2010," Judge Denise Cote said on Wednesday.
A spokesman for the tech giant, Tom Neumayr, said Apple will continue to fight "false allegations" and plans to appeal against the ruling.
Judge Cote said: "The plaintiffs have shown that the publisher defendants conspired with each other to eliminate retail price competition in order to raise e-book prices, and that Apple played a central role in facilitating and executing that conspiracy."
The US Department of Justice, which is pursuing Apple in the case, said that the conspiracy was designed to challenge Amazon's dominance of the e-book market.
Assistant Attorney General Bill Baer, declared the ruling "a victory for millions of consumers who choose to read books electronically."
"This decision by the court is a critical step in undoing the harm caused by Apple's illegal actions," he added.
The judge has ordered a new hearing to determine the extent to which Apple will be fined for the discrepancy.
Penguin settled its case for $75 million (£49 million), Macmillan settled for $26m (£17.4 million). Hachette, HarperCollins and Simon & Schuster created a $69 million (£46.2 million) fund for consumer refunds.