BOSTON — Workers will go on strike at 39 Shaw’s supermarkets in Massachusetts and Rhode Island and at the distribution center Sunday if a contract agreement is not reached, a union official said.

Shaw’s said the stores will continue operating if there is a strike.

Strike issues are pay raises, health care payments, time off and job security, said Russell Regan, president of United Food and Commercial Workers Local 791.

The present contract expires Sunday morning, and he said strike assignments will be determined if members do not ratify a new contract at a union meeting Sunday afternoon.

Negotiations have gone on for four months. Union members voted to authorize a strike July 13.

Bernard Rogan, a spokesman for Shaw’s, said the company wrote to employees indicating it will hire replacement workers if there is a strike. About 80 percent of the employees are part time, the company said.

“We are prepared to keep our stores open and provide the same level of service that our customers are accustomed to,” Rogan told the Boston Herald.

“We’ve always bargained over the level of benefits workers should receive. Now the company wants to bargain over concessions,” Regan told The Boston Globe.

Rogan said the Shaw’s supermarkets in Maine will not be affected by the strike as “distribution is adequately covered.”

None of the Shaw’s employees in Maine are unionized.

The union wants pay raises for all employees of the 39 unionized markets and the warehouse in East Bridgewater, and said it also is concerned that the company wants to drop a job security clause.

The company wants to increase copayments for doctors’ visits from $5 to $10, and the union said a clause would have workers pay 50 percent of future premium increases.

Rogan said the company wants to reduce the number of sick days from nine to five and wants to require workers who leave the company to return vacation pay, depending on when they leave and how much vacation they’ve taken.

Shaw’s has 119 stores in New England, including 38 in Massachusetts and 11 in Rhode Island. All the Rhode Island stores are unionized; 28 in Massachusetts are.

Shaw’s is headquartered in East Bridgewater. The company is owned by J. Sainsbury’s of London, England.