The agency said eliminating Saturday mail service represented a substantial cost savings because of fewer staff hours and less equipment needed to maintain the deliveries.

The Postal Service also said the rise in online retail purchases and other e-commerce was contributing to its increase in that area and was why it would continue to deliver packages on Saturdays.

Since 1981, a Congressional mandate has required the Postal Service to deliver mail six days a week. But on Wednesday the agency argued that since the current stopgap budget measure for the entire government, known as a continuing resolution, did not contain language explicitly mandating six-day delivery, the agency could make the changes without Congressional approval.

But some members of Congress immediately questioned the Postal Service’s claim.

“The passage of the continuing resolution did not suspend that language, as they claim, but in fact extended it,” said Representative José E. Serrano, Democrat of New York and ranking member on the appropriations subcommittee on financial services and general government, which has jurisdiction over the post office. “Rather than use very dubious legal arguments to end Saturday delivery, the U.S.P.S. should work hand in hand with Congress to come up with a successful restructuring and reform package that allows them to become more efficient while maintaining vital services like Saturday delivery.”

Senator Thomas R. Carper, Democrat of Delaware, who led a bipartisan effort to pass a postal overhaul bill last year, called the post office decision disappointing. But he added, “Despite my disappointment, it’s hard to condemn the postmaster general for moving aggressively to do what he believes he can and must do to keep the lights on at the Postal Service, which may be only months away from insolvency.”

Last April, the Senate passed a bill that provided early retirement incentives to about 100,000 postal workers, or 18 percent of its employees, and allowed the Postal Service to recoup more than $11 billion it overpaid into an employee pension fund. The Senate bill did not stop Saturday deliveries immediately, but it would have allowed the agency to consider the issue in two years.