WASHINGTON » The struggling U.S. Postal Service today reported an annual loss of a record $15.9 billion and forecast more red ink in 2013, capping a tumultuous year in which it was forced to default on billions in payments to avert bankruptcy.

The financial losses for the fiscal year ending Sept. 30 were more than triple the $5.1 billion loss in the previous year. Having reached its borrowing limit, the mail agency is operating with little cash on hand, putting it at risk in the event of an unexpectedly large downturn in the economy.

“It’s critical that Congress do its part and pass comprehensive legislation before they adjourn this year to move the Postal Service further down the path toward financial health,” said Postmaster General Patrick Donahoe, calling the situation “our own postal fiscal cliff.”

Much of the red ink in 2012 was due to mounting mandatory costs for future retiree health benefits, which made up $11.1 billion of the losses. Without that and other related labor expenses, the mail agency sustained an operating loss of $2.4 billion, lower than the previous year.

Donahoe said the post office has been able to reduce costs significantly by boosting worker productivity. But he said the mail agency has been hampered by congressional inaction on a postal overhaul bill that would allow it to eliminate Saturday mail delivery and reduce its $5 billion annual payment for future health benefits.

“We cannot sustain large losses indefinitely. Major defaults are unsettling,” said Donahoe, who made clear that the Postal Service would now be profitable had Congress acted earlier this year.

Earlier this year, the post office defaulted on two of the health prepayments for the first time in its history.

The Postal Service, an independent agency, does not receive tax money for its day-to-day operations but is subject to congressional control.

The Senate passed a postal bill in April that would have provided financial relief in part by reducing the annual health payments and providing a multibillion-dollar cash infusion, basically a refund of overpayments the Postal Service made to a federal pension fund. The House, however, remains stalled over its own legislation that would allow for aggressive cuts, including an immediate end to Saturday delivery.

It remained unclear whether House leadership would take up the postal bill in its current lame-duck session. Rural lawmakers are resisting action, worried about closures of postal facilities in their communities. Congress is focused now on a Jan. 1 deadline to avert across-the-board tax increases and spending cuts known as the “fiscal cliff.”

While urging quick congressional action, the Postal Service acknowledged the uncertainty in its legal filings today, which anticipate that Congress will fail to act. But Rep. Darrell Isa, chairman of the House Oversight and Government Reform Committee and sponsor of the House bill, has said he believes postal legislation can be passed this year.

“The U.S. Postal Service is clearly marching toward a financial collapse of its own,” said Sen. Tom Carper, D-Del., a sponsor of the Senate bill. “I am hopeful that now that the elections are over, my colleagues and I can come together and pass postal reform legislation so that a final bill can be signed into law by the end of the year.”

Overall, the post office had operating revenue of $65.2 billion in fiscal 2012, down $500 million from the previous year. Expenses climbed to $81 billion, up from $70.6 billion, largely due to the health prepayments. The annual payment of roughly $5.6 billion had been deferred for a year in 2011, resulting in a double payment totaling $11.1 billion that became due this year. The Postal Service is the only government agency required to make such payments.

The post office also has been rocked by declining mail volume as people and businesses continue switching to email and other online options in place of letters and paper bills. The number of items mailed in the last year was 159.9 billion pieces, a 5 percent decrease. Much of the decline came in first-class mail.

On the plus side, the mail agency reported that its fast-growing shipping services, which include express and priority mail, grew by 9 percent, helping to offset much of the declining revenue from first-class mail. Donahoe said package volume also is expected to jump by 20 percent this holiday season compared to the same period last year, boosted by increased consumer purchases on e-Bay, Amazon.com and other Internet shopping sites.

Joseph Corbett, chief financial officer for the Postal Service, said the mail agency expects to operate for the first half of next year with about four days of cash reserves, a low amount which he described as unheard of for any well-run business. Cash levels dipped perilously close to zero last month before bouncing higher due to a surge in election-related mail. In all, campaign mailings and mail-in ballots helped bring in $500 million, a new high and roughly double the amount in the 2008 election year.

“We are far short of liquidity,” Corbett said.

Last month, the post office said it will increase postage rates on Jan. 27, including a 1-cent increase in the cost of first-class mail, to 46 cents. The rate increase, which is tied to the rate of overall inflation, will make only a small dent in financial losses.

The Postal Service also originally sought to close low-revenue post offices in rural areas to save money, but after public opposition, it is now moving forward with a new plan to keep 13,000 of them open with shorter operating hours.

Without legislative changes, it said, annual losses will exceed $21 billion by 2016.

“If Congress fails to act, there could be postal slowdowns or shutdowns that would have catastrophic consequences for the 8 million private sector workers whose jobs depend on the mail,” said Art Sackler, co-coordinator of the Coalition for a 21st Century Postal Service, a group representing the private sector mailing industry.