WASHINGTON – The U.S. Postal Service is bracing for a first-ever default on billions in payments due to the Treasury, adding to widening uncertainty about the mail agency’s solvency as first-class letters plummet and Congress deadlocks on ways to stem the red ink.

With cash running perilously low, two legally required payments for future postal retirees’ health benefits – $5.5 billion due Wednesday, and an additional $5.6 billion due in September – will be left unpaid, the mail agency said Monday. Postal officials said they also are studying whether they may need to delay other obligations.

The defaults won’t stir any kind of catastrophe in day-to-day mail service. Post offices will stay open, mail trucks will run, employees will get paid, current retirees will get health benefits.

But a growing chorus of analysts, labor unions and business customers are troubled by continuing losses that point to deeper, longer-term financial damage.

Postmaster General Patrick Donahoe has described a “crisis of confidence” that could lead once-loyal customers to abandon the mail.

“I think for my generation it was a great asset – if you had a letter or package and you needed it to get up to the North Pole, you knew it would be delivered,” said Jim Husa, 87, of Lawrence, Mich., after mailing letters recently at a post office. Noting the mail agency’s financial woes, he added: “Times have changed, and we old-timers know that. FedEx and UPS and the Internet seem to be making the Postal Service obsolete.”

Banks are promoting electronic payments, citing in part the growing uncertainty of postal mail. The federal government will stop mailing paper checks starting next year for millions of people who receive Social Security and other benefits, paying by direct deposit or debit cards instead.

First-class mail volume, which has fallen 25 percent since 2006, is projected to drop an additional 30 percent by 2016.

The Postal Service estimates it is losing $25 million a day, which includes projected savings it had expected to be accruing by now if Congress this spring had approved its five-year profitability plan. That plan would cut Saturday delivery, reduce low-volume postal facilities and end its obligation to pay more than $5 billion each year for future retiree health payments.

“This could be a watershed event to motivate consumers and businesses to stop writing checks,” said Rodney Gardner, head of global receivables at Bank of America.