ST. LOUIS (AP) _ Machinists working for the Boeing Co. in St. Louis voted Sunday to reject a contract offer and authorize a strike if negotiations with the company don't resume.

Union officials were contacting Boeing representatives Sunday afternoon to determine if they would go back to the bargaining table, said Bruce Darrough, a spokesman for the International Association of Machinists and Aerospace Workers District 837.

If there is a strike, it could not begin until May 21 because the union is required to give the company seven days notice, Darrough said.

The vote on the three-year contract offer was 1,857 against and 497 for, or 79 percent of the membership voting Sunday, Darrough said. A second vote was 1,659 for a strike and 543 against.

Boeing gave the machinists a three-year contract offer Saturday for the 3,200 St. Louis employees. Their contract expired Sunday.

The same union conducted a 99-day strike in 1996 before ratifying the last contract. Darrough said some union leaders were surprised by Sunday's vote.

Job security and retirement benefits offered in the contract were the primary concerns for union members, Darrough said. The company has proposed merging several classifications of workers, which raised concerns about workers having to do several different jobs, he said.

On Saturday, the company said the contract proposal was fair and competitive.

``It addresses each and every major concern that the IAMAW has raised on behalf of its members, most notably job security, wage increases and retirement benefits,'' Boeing site leader John Van Gels said.