BOSTON – Shaw’s Supermarkets Inc. and a union representing about 7,000 of its employees agreed to a three-year contract on Sunday, averting a strike.

Officials from the supermarket chain and United Food and Commercial Workers Local 791 reached a tentative agreement early Sunday morning after marathon talks. Union members in Massachusetts voted in the contract in the afternoon, and Wells, Maine, members approved it later in the day, negotiators said.

Union spokesman Peter Derouen said members approved the contract by “a comfortable margin.”

“We were able to get a pretty decent contact ratified,” Derouen said. “We’re glad we could maintain one of best retail contracts in the industry.”

The new contract includes wage and pension increases, and compares favorably with other contracts on salary, benefits and working conditions, he said. Company and union negotiators also reached an agreement on a severance package for workers at the East Bridgewater Distribution Center set to close in October.

The new contract will expire in 2004.

The agreement was “borne out of mutual respect for our employees and commitment to reach a common goal,” said Shaw’s CEO Ross McLaren.

Negotiations on the contract, covering about 7,000 retail workers at 38 supermarkets in Massachusetts and Rhode Island, continued for more than six weeks. The expiring contract was negotiated in 1997.

The contract was set to expire at midnight Friday, but was extended so talks could continue. Workers had already approved a strike vote.

Shaw’s, based in West Bridgewater, is the second-largest grocery retailer in New England. A subsidiary of British grocery conglomerate J. Sainsbury, it operates about 170 stores throughout New England. It grew considerably in the Boston area with its 1998 purchase of the Star Market chain.