WASHINGTON – Facing bankruptcy, the U.S. Postal Service is pushing ahead with unprecedented cuts to first-class mail next spring that will slow delivery and, for the first time in 40 years, eliminate the chance for stamped letters to arrive the next day.

The estimated $3 billion in reductions, to be announced in broader detail today, are part of a wide-ranging effort by the cash-strapped Postal Service to quickly trim costs, seeing no immediate help from Congress.

The changes would provide short-term relief, but ultimately could prove counterproductive, pushing more of America’s business onto the Internet. They could slow everything from check payments to Netflix’s DVDs-by-mail, add costs to mail-order prescription drugs, and threaten the existence of newspapers and time-sensitive magazines delivered by postal carrier to far-flung suburban and rural communities.

That birthday card mailed first-class to Mom also could arrive a day or two late, if people don’t plan ahead.

The cuts, now being finalized, would close roughly 250 of the nearly 500 mail processing centers across the country as early as next March. Because the consolidations typically would lengthen the distance mail travels from post office to processing center, the agency also would lower delivery standards for first-class mail that have been in place since 1971.

Currently, first-class mail is supposed to be delivered to homes and businesses within the continental United States in one day to three days. That will lengthen to two days to three days, meaning mailers no longer could expect next-day delivery in surrounding communities. Periodicals could take between two days and nine days.

About 42 percent of first-class mail is now delivered the next day. An additional 27 percent arrives in two days, about 31 percent in three days and less than 1 percent in four days to five days. After the change next spring, about 51 percent of all first-class mail is expected to arrive in two days, with most of the remainder delivered in three days.

The consolidation of mail processing centers is in addition to the planned closing of about 3,700 local post offices. In all, roughly 100,000 postal employees could be cut as a result of the various closures, resulting in savings of up to $6.5 billion a year.

Expressing urgency to reduce costs, Postmaster General Patrick Donahoe said in an interview that the agency has to act while waiting for Congress to grant it authority to reduce delivery to five days a week, raise stamp prices and reduce health care and other labor costs.

The Postal Service, an independent agency of government, does not receive tax money, but is subject to congressional control on large aspects of its operations. The changes in first-class mail delivery can go into place without permission from Congress.

After five years in the red, the post office faces imminent default this month on a $5.5 billion annual payment to the Treasury for retiree health benefits.

It is projected to have a record loss of $14.1 billion next year amid steady declines in first-class mail volume. Donahoe has said the agency must make cuts of $20 billion by 2015 to be profitable.

The post office already has announced a 1-cent increase in first-class mail to 45 cents beginning Jan. 22.