SHREWSBURY — The Central Massachusetts Mail Processing and Distribution Center on Main Street is among as many as 82 United States Postal Service facilities that the agency announced Monday would be consolidated, beginning in January.

John H. Flattery, president of the American Postal Workers Union AFL-CIO Local 4553, which represents workers at the plant, said the announcement that the Postal Service will resume the network consolidations was the last thing he expected to hear.

"Between January 2015 and September 2015, they will be shutting down the processing center in Shrewsbury. It's just asinine as far as I'm concerned," Mr. Flattery said. "We just converted a large number of temporary employees to career employees. When this process is completed, they said they are going to get rid of the plant and move the employees to some place else or lay them off."

He said the 400 to 500 employees at the facility were "stunned" when they were informed Monday afternoon.

Under the plan, Mr. Flattery said, the Shrewsbury facility will close and all letter mail will be sent to the Boston facility to be processed. The flats with large envelopes, magazines and catalogs will be processed at the facility in North Reading.

The 260,000-square-foot facility here was built in 1991 on 51.7 acres at 192 Main St. It prepares mail for delivery to about 93 post offices that serve more than 500,000 residences and businesses each day.

In 2011, the plant was among several throughout the country that the cash-strapped U.S. Postal Service considered closing and consolidating operations. Some 141 plants nationwide were consolidated in the first phase in 2012 and 2013. In 2012, the agency said the Shrewsbury center would remain open at least until 2014, when the final phase of plant consolidations and closures would take place. The final phase of closures was then scheduled to begin last February.

Postmaster General Patrick R. Donahoe, in his announcement Monday, said the highly successful consolidation in 2012 and 2013 "resulted in negligible service impact, generating annualized cost savings of $865 million and required no employee layoffs." He said next year's closures and consolidations will generate an additional $750 million in annual savings.

"The Postal Service has recorded substantial losses over the last three years and continues to see steep declines in first-class mail volume and revenue. As a result, we find ourselves with excess capacity in the network and few alternatives to reduce costs. Our operating costs are continuing to increase, and our debt and other liabilities threaten our financial viability," Mr. Donahoe said in a video message to employees. He said the agency is committed to finding affected employees reassignments in other locations.

Mr. Flattery said the Postal Service has an operating surplus of $1.3 billion this year. But it is still burdened by the 2006 Postal Accountability and Enhancement Act. Under this legislation, the USPS, unlike any other federal agency, is required to prepay future retiree health benefits, to the tune of about $5.5 billion a year.

He said he believes Mr. Donahoe's announcement on Monday is another attempt to try to force Congress to pass legislation he thinks will benefit the agency.

"Congress sits back and does nothing. Congress is creating this crisis," he said. "We're fortunate to have a great delegation from Central Massachusetts. But the right wing of the GOP is hell bent on destroying the service. It's just mind-boggling."

Contact Elaine Thompson at elaine.thompson@telegram.com.

