WASHINGTON — The Postal Service on Thursday received another setback in its attempts to stop delivering mail on Saturdays.

The Government Accountability Office, a nonpartisan investigative arm of Congress, said in a legal opinion that the post office did not have the authority to make the change without Congressional approval, based on a spending measure passed by Congress last year.

The opinion was issued on the same day the House sent the president a spending measure that also required the post office to maintain Saturday delivery. That measure keeps the government operating through Sept. 30.

In a statement, the Postal Service said it disagreed with the accountability office. “The opinion does not address the Postal Service’s proposal to move to five-day mail delivery, with six-day package delivery, during the week of Aug. 5,” said David Partenheimer, a spokesman for the agency.