About a dozen post offices in southern Maine received a late delivery of mail Wednesday because of operational changes at the Scarborough distribution center.

One post office in Portland didn’t receive any mail at all.

A spokeswoman for the U.S. Postal Service apologized for the lapse in service and promised that the delivery of first-class mail would return to a normal schedule Thursday.

The post office in downtown Portland, which primarily serves box holders in the vicinity of One City Center, did not get any mail Wednesday. The postal service was unable to identify how many customers received a late delivery or no mail at all.

“We apologize to our customers for any inconvenience,” Melissa Lohnes, a postal service spokeswoman based in Boston, said Wednesday. “We expect that everyone will receive their mail tomorrow.”

In the past, the distribution center in Scarborough, which employs about 500 people, used three shifts to sort local first-class mail and make sure it arrived in one day. But on Saturday, Lohnes said, Scarborough started using a “skeleton crew” on the overnight shift and moved nearly all of its workforce to the two daytime shifts.

The shift changes led to delays in mail being processed as employees learned new tasks and adjusted to new schedules, she said.

“You are going to see a lot more mail processed during the day. Everything has been reversed,” Lohnes said.

The switch stems from a new nationwide service standard implemented this month that reallocates postal service resources to address changes in customer demand, she said. Instead of focusing on processing first-class mail overnight, postal service workers are concentrating their hours and energy on expediting package deliveries.

“Our first-class mail has been in decline for more than a decade,” Lohnes explained. “A lot of our customers are paying bills online or sending electronic greeting cards.”

The postal service’s so-called “Network Rationalization” plan also will lead to the consolidation of 82 mail-processing facilities across the country in 2015. The centers in Scarborough and Bangor won’t be affected, Lohnes said. According to the postal service, consolidation and implementing efficiencies will save it about $750 million a year.

Some postal service workers in Maine don’t like the changes.

Tim Doughty, who works at the Scarborough facility, said a letter mailed in Portland to another Portland address should arrive in one day.

“Yesterday when I walked through the building (in Scarborough) there was a lot of mail just sitting there,” said Doughty, who is president of the southern Maine American Postal Workers Union.

“Every American is entitled to a universal service. That’s why we deliver the mail by train, plane, dogsled and automobile,” Doughty said. “Why the postmaster general wants to slow it down makes no sense to me. Delivering the mail on time is the bread and butter of the post office.”

Paul Brawn, who works as a window clerk in the Augusta post office, said the change in the priorities of how first-class mail is delivered is disturbing. He doesn’t like the idea that a letter mailed on Monday in Augusta to an Augusta address won’t be delivered until Wednesday at the earliest.

“The postal service is trying to save money, but I don’t see how that is going to work when they are pushing customers away,” Brawn said.

Two tenants of One City Center did not receive any mail Wednesday.

David Gilman, a vice president of finance for MaineToday Media, which owns the Portland Press Herald, said the newspaper has experienced a decline in mail volume for three straight days and got none on Wednesday. The company relies on daily mail deliveries of bill payments to its post office box, Gilman said.

Harold Pachios, a partner at the Preti Flaherty law firm, said his office still receives a significant amount of first-class mail that includes court documents and communications from clients. But Pachios was not overly concerned about Wednesday’s disruption provided the postal service gets deliveries back on track.

“If this had happened 25 years ago before emails, it would have been a major problem, but now we do get a lot of correspondence electronically,” Pachios said.