WASHINGTON — The U.S. Postal Service said Tuesday it has lost $5.1 billion in the past year, pushing it closer to imminent default on a multibillion-dollar payment and to future bankruptcy as the weak economy and increased Internet use drive down mail volume.

The financial losses for the year ended Sept. 30 came despite deep cuts of more than 130,000 jobs in recent years and the closing of some smaller local post offices.

Losses will only accelerate in the coming year, Postmaster General Patrick Donahoe warned, citing faster-than-expected declines in first-class mail. He implored Congress to take swift, wide-ranging action to stabilize the ailing agency’s finances as it nears a legal deadline Friday to pay $5.5 billion into the U.S. Treasury for future retiree health benefits.

Congress is expected to grant a reprieve, but that will only delay the day of reckoning for an agency struggling for relevance in an electronic age. Based on current losses, the Postal Service says it will run out of money — or come dangerously close — next September, forcing it to halt service.

“We are at a point where we require urgent action,” Donahoe said.

In the event of a shutdown, private companies such as FedEx and UPS could handle a small portion of the material the post office moves, but they do not go everywhere. No business has shown interest in delivering letters everywhere in the country for a set rate of 44 cents for a first-class letter.

For the fiscal year ended Sept. 30, the post office had income of $65.7 billion, down $1.4 billion from the previous year. Expenses totaled $70.6 billion.

The loss of $5.1 billion was less than a previous estimate of $10 billion, but only because the $5.5 billion payment — originally due Sept. 30 — was deferred until Nov. 18 with the approval of Congress. In 2010, losses totaled $8.5 billion.

Mail volume this past year totaled 168 billion pieces, compared with 171 billion in 2010, a decline of 1.7 percent. At the same time volume was declining, the post office had to begin service to thousands of new ad- dresses to accommodate population growth and new businesses.

The Postal Service, an independent agency of government that does not receive tax money for its operations, is not seeking federal funds. Instead, postal officials want changes in the way they operate so they can save money.

They have asked Congress for permission to reduce mail delivery to five days a week, which many lawmakers oppose, and to eliminate or reduce the annual payments of about $5.5 billion to pre-fund retiree health benefits. The agency also wants the return of at least $6.9 billion it says was overpaid into federal retirement funds.

The service also seeks more layoffs, which are barred by current contracts with its employee unions, and the authority to negotiate with unions on a possible alternate health-care system that would cost less.

Congress is expected to pass a stop-gap spending measure this week that would extend Friday’s payment deadline until mid-December.

The Postal Service has said a short-term delay of the $5.5 billion payment won’t change its grim forecast of possible bankruptcy next year. Officials also said Tuesday that the proposed legislation currently falls short in reducing health care costs and authorizing immediate five-day-a-week delivery.

“We’re hoping for long-term, comprehensive legislation that will solve the issue and make other changes so the Postal Service can be profitable again — not have more delays that just kick the can down the road,” postal spokesman David Partenheimer said.

A recent Quinnipiac poll found that 79 percent of registered voters were in favor of ending Saturday deliveries to help with the agency’s financial problems.