WASHINGTON—In an unusual high-stakes move, Staples Inc. and Office Depot Inc. on Tuesday said they wouldn’t present a defense against the government’s challenge to their planned merger, arguing that the Federal Trade Commission’s case against the deal was so weak that a judge should simply rule in their favor.

Staples lawyer Diane Sullivan told a federal judge that court proceedings so far had shown the FTC’s case to be fatally flawed, with the commission unable to carry its burden of proof that the merger was likely to harm competition.

“Based on the evidence, they’ve fallen woefully short,” she said in a packed courtroom filled with onlookers surprised by the companies’ bold legal move.

U.S. District Judge Emmet G. Sullivan has spent the past two weeks hearing the FTC’s case against the merger. He, too, said Tuesday that he hadn’t expected the companies to rest their case without calling any defense witnesses.

“I did not anticipate this,” said Judge Sullivan, who is likely to rule in the next few weeks. He ordered the parties to return to his courtroom on April 19 for a final round of arguments before he makes his decision. He also encouraged the parties to try to find a way to settle the case if they can. Past settlement discussions between the two sides have been unsuccessful.