In a decision that could spur an e-book price war in the publishing industry, a federal judge has approved a settlement between the U.S. Department of Justice (DOJ), and Hachette Book Group, HarperCollins and Simon & Schuster in a civil antitrust case that accused the companies of publishers' price-fixing digital books, the New York Times reported.

The Sept. 5 ruling by Denise Cote, the federal district judge in Manhattan, N.Y., overseeing the case, comes a week after the three major publishers reached a deal for more than $70 million with 54 states and U.S. territories to settle their price-fixing claims. Also still to come before Cote, that settlement has yet to be approved.

In her ruling, Cote called the matter a "straightforward price-fixing case," of which the settlement had been widely expected to be approved, the Times reported. The ruling promised to empower Amazon to drop the price of many e-books back to $9.99 or lower in the coming months, a move that could pressure competing retailers to do the same.

The DOJ filed the antitrust suit in April against computer giant Apple and five publishers—Hachette, HarperCollins, Simon & Schuster, Penguin Group (USA) and Holtzbrinck Publishers (Macmillan)—to allow greater competition on e-book prices. Hachette, HarperCollins and Simon & Schuster denied wrongdoing, but agreed to settle with the government. However, Penguin, Holtzbrinck and Apple declined a settlement, so the companies face a trial next summer.

The DOJ said that the defendants sought to raise e-book prices "significantly higher" than the $9.99 level at which they were selling, Bloomberg reported. The publishers viewed Amazon.com's discounted prices on e-books as a "substantial challenge to their traditional business model," according to the complaint.

Hachette publishes Christian books under its FaithWords and Jericho Books imprints. HarperCollins is the parent company of Zondervan and Thomas Nelson. Simon & Schuster publishes Christian titles under Howard Books.

The settlement was criticized by Barnes & Noble, the Authors Guild and the American Booksellers Association.

Paul Aiken, executive director of the Authors Guild, told the Times that the biggest losers would be traditional independent bookstores. If the cost of a newly released e-book drops further, the bookstores will have more trouble selling hardcover books at their current prices, he said.

When the settlement was first announced, Amazon called it "a big win for Kindle owners," and said it looked forward to eventually lowering its prices on e-books.