The U.S. Postal Service will stop processing mail in Hampden and move that operation to a processing department in Scarborough. The move is part of a nationwide consolidation of mail processing operations, the Postal Service announced on Thursday.

Most of the 183 jobs at the Eastern Maine Processing and Distribution Facility in Hampden will be affected, spokesman Tom Rizzo said.

He said many of those employees will be offered jobs in eastern Maine post offices or at the Southern Maine Processing and Distribution Center in Scarborough.

“We will work with the unions to find their members postal positions elsewhere,” he said.

Rizzo said he didn’t know how many new jobs would be added at the Scarborough plant, which currently employs 540 people. He said it will take the Postal Service several weeks to figure out the impact.

The Hampden plant will remain open as a transportation hub for the transfer of mail coming from and going to southern Maine, he said.

Rizzo said no date for the move has been set, but it cannot happen before May 15 — the end date for a moratorium on closing and consolidating post offices and processing facilities, enacted in December to allow officials time to form a plan.

The Postal Service is closing 223 of its 460 mail processing departments nationwide as part of effort to reduce cost. The Postal Service since 2006 has seen 25 percent decline in the volume of first-class mail.

To preserve the long-term affordability of mail and to return to financial stability, the Postal Service said it is needs to reduce costs by $20 billion by 2015.

The Postal Service pointed out that it receives no tax dollars for its operations and that it relies instead on the sale of postage, postal products, and services.

“Consolidating operations is necessary if the Postal Service is to remain viable to provide mail service to the nation,” Chief Operating Officer Megan Brennan said in the prepared statement.

U.S. Sen. Susan Collins, R-Maine, the ranking Republican on the Homeland Security and Governmental Affairs Committee, is urging Postmaster General Patrick Donahoe to reconsider the decision to close the Hampden plant.

In a letter she sent to him on Wednesday, she said she was “deeply disappointed and shocked” by the decision.

“If mail to and from the northern half of Maine has to travel all the way to the Scarborough plant to be processed, longer delivery times are inevitable, and that has consequences — for small businesses advertising their products or billing their customers, for families who use the mail for their daily newspaper delivery, for seniors who rely on the mail for their prescription drugs, and for so many others.”

Collins, in a statement, said that the changes will threaten the Postal Service’s revenues by driving customers to use the Internet or alternative shippers.

“Changes that lead to further declines in volume would only worsen the crisis facing the Postal Service and harm our economy since so many jobs depend on a healthy Postal Service,” Collins said.