The U.S. Postal Service on Wednesday moved to circumvent Congress by announcing that it will stop collecting and delivering mail on Saturdays in an effort to stem its staggering financial losses.

The cutback in service to Monday-through-Friday delivery, starting Aug. 5, would result in a $2 billion savings annually for the agency, which lost $15.9 billion in the last fiscal year. Packages will still be delivered on Saturdays, and post office boxes will continue to receive mail that day. Post offices will also be open to sell stamps and other materials. But magazines, some newspapers, catalogues and Netflix movies would not reach customers’ homes on Saturdays.

Postmaster General Patrick R. Donahoe, citing the decline in mail volume caused by the Internet as Americans switch to communicating and paying bills online, said five-day service will help keep the mail agency solvent.

“It’s irresponsible for us not to pursue this course,” he said. “It’s too big of a cost savings to ignore.”

The change would defy lawmakers’ prohibition on abolishing six-day service, a 150-year-old hallmark of the mail system. For years, Congress has rejected the Postal Service’s call for five-day delivery. But Donahoe saw an opportunity in the stopgap budget that is funding the government through March 27. By the time it expires, he believes momentum will be on his agency’s side and Congress may be forced to yield.

An end to Saturday mail

Since 1983, the annual appropriations bill giving federal agencies spending authority has contained a rider requiring the Postal Service to deliver the mail six days a week. But Donahoe said he can make the change unilaterally and bypass lawmakers.

He hinted that the current stopgap measure, which includes the prohibition on five-day service, may not be binding anyway. Postal officials are gambling that Congress will keep it out of any future spending measure lawmakers approve.

“We think we’re on good footing with this,” Donahoe said. Asked what he would do if Congress continued to pass legislation blocking five-day delivery, he said, “Let’s see what happens.”

“This is not like a gotcha or anything,” he said.

Postal officials’ proposal to end Saturday delivery has met resistance from lawmakers in rural districts and those supported by labor unions, who oppose the change and are avid political donors.

On Wednesday, union officials again condemned the plan. Letter carriers, clerks and mail sorters would lose jobs — about 20,000 to 25,000 — through attrition, reassignments and possible buyouts, officials said. As many as 12 percent of city letter carriers routinely receive overtime pay; much of that would also disappear by eliminating Saturday delivery.

“This maneuver by Mr. Donahoe flouts the will of Congress, as expressed annually over the past 30 years in legislation that mandates six-day delivery, which remains in effect today,” Fredric Rolando, president of the National Association of Letter Carriers, said in a statement.

The executive board of the union representing rural carriers on Wednesday cast a vote of “no-confidence” in Donahoe. At least one union said it plans to sue over the change.

Many details of the postal plan are vague. Postal officials said most short-distance letters mailed on a Friday would still arrive on a Monday and mail scheduled for delivery Tuesday after a three-day weekend would arrive on schedule. But some were skeptical that such a big change, as the agency closes hundreds of mail-sorting hubs, would not delay deliveries.

The announcement comes as the agency continues to hemorrhage money, much of it because of a 2006 law requiring the Postal Service to set aside $5.5 billion a year for health benefits for future retirees. The agency defaulted twice on the payments last year after reaching its legal borrowing limit. It has closed hundreds of mail-sorting hubs, reduced hours at thousands of post offices and reduced the workforce by 193,000 to cut costs.

White House press secretary Jay Carney declined to offer a definitive opinion on the plan, saying that the Obama administration only learned of the decision Tuesday and that the Postal Service is independent. He said the White House prefers a comprehensive package of reforms, and faulted the Republican-led House for the lack of movement.

Postal officials say they have broad public support for their plan. They cite a Postal Service study showing that about seven in 10 Americans support the switch to five-day delivery.

But support in Congress is not guaranteed — nor does opposition fall along partisan lines. House and Senate leaders were unable to reach consensus in the last Congress on legislation to stanch the losses. The Senate passed a bill that would have delayed five-day delivery for two years, among other changes. A more aggressive House bill that would have allowed five-day service to proceed did not reach the floor.

Republicans who supported that bill applauded Wednesday’s announcement.

“Supporting the U.S. Postal Service’s plan to move forward with five-day mail delivery is one such solution worthy of bipartisan support,” Rep. Darrell Issa (R-Calif) and Sen. Tom Coburn (R-Okla.) said in joint statement.

But Sen. Thomas R. Carper (D-Del.), chairman of the committee that oversees postal operations, said he was disappointed. “For nearly three decades, it has been the clear intent of Congress that the Postal Service provide most communities with six days of mail delivery,” he said in a statement.

“It’s hard to condemn the Postmaster General for moving aggressively for doing what he believes he can and must do to keep the lights on at the Postal Service,” Carper added. He also criticized Congress for failing to agree on a broader set of reforms.

House Speaker John A. Boehner (R-Ohio) expressed hope that lawmakers would make progress on legislation to overhaul the Postal Service.

The change could be felt most in rural areas, where remote communities rely heavily on mail delivery.

“This is going to be very rough on Vermont and rural America, but it’s an uphill challenge to reverse it,” said Rep. Peter Welch (D-Vt.). “Our political prospects are uphill if not up-mountain.”

Among postal customers, opinions were divided Wednesday.

Richard Findley, 67, who lives near the Manassas Post Office, said the only thing he receives in the mail these days are bills. Personal notes and cards have long migrated to e-mail.

“I can’t see why a post office even operates on Saturday,” he said. “Nobody really needs mail.”

But outside the District post office at 20th Street and Florida Avenue NW, Dwayne Reed, an electrician who lives in Brookland, said he is concerned for the people who rely on checks in the mail.

“If you’re receiving a check on the first of the month from the government for Social Security or unemployment, the first may be a Saturday,” he said. “Those extra two days of waiting may be critical for people who rely on those checks for food.”

Anthony Duckett, who lives in Clinton and owns a commercial cleaning business, said the plan is unfair to customers.

“They are increasing Postal Service prices but decreasing the services they are offering,” said Duckett, who added that he receives most of his invoices and billing statements in the Saturday mail.

“That would put me a day or two behind in receiving it . . . so it would affect me greatly,” he said.

While first-class mail has always been the Postal Service’s biggest moneymaker, packages are a growing profit center. But they do not include catalogues, magazines, movies and small newspapers that rely on the post office for home delivery. Many of those mailers will probably adjust their delivery days.

About a third of the 4,000 newspapers represented by the National Newspaper Association rely on the post office for home delivery.

“Newspapers can deliver with contract carriers, but that requires setting up and managing routes, not to mention monitoring the contractors to make sure they do the job properly,” spokesman Stan Schwartz said. “Some newspapers just aren’t prepared to handle it.”

Netflix, the online movie rental service, said in a statement that ending Saturday delivery could make the company more profitable by lowering costs as subscribers move from DVDs to watching video streamed over the Internet.

The Postal Service’s annual loss for the fiscal year ended Sept. 30 was the largest in the agency’s history. It is scheduled Friday to release financial results for October through December, typically its most profitable quarter.

Josh Hicks, Jeremy Borden, Alex Rudansky and Ovetta Wiggins contributed to this report.