HOUSTON (Reuters) - Striking workers at BP Plc's (>> BP plc) Whiting, Indiana, refinery ratified a new four-year contract on Monday, formally ending a three-month work stoppage, said the president of the United Steelworkers local chapter representing the hourly employees.

The workers are scheduled to begin returning to work on May 18 in staggered groups, said USW Local 7-1 President Dave Danko.

"In the next couple of weeks, all we want to focus on is making a safe transition at the refinery," Danko said.

BP has kept the refinery in operation using temporary replacement workers since Feb. 8 when USW members walked out as part of the largest strike by U.S. refinery and chemical plant workers in 35 years.

Jorge Lanza, the Whiting refinery manager, said the company looks forward to the workers' return beginning on May 18. "Our reintegration process is expected to take about three weeks, through an orderly, smooth process that focuses on safety and promotes teamwork."

The ratification was expected following agreement on Friday between BP and Local 7-1 on the terms of bringing the 1,060 union refinery workers back to their jobs. The two sides reached a tentative contract agreement on May 1.

Danko said the vote was 92 percent in favour of the new contract, which provides a 2.5 percent pay increase in the first year, a 3 percent increase in the second and third years and a 3.5 percent boost in the fourth year.

The national strike eventually expanded to workers at 15 plants including 12 refineries that account for one-fifth of U.S. capacity. It began after talks broke down between negotiators for the USW International union and U.S. refinery owners over tighter standards to prevent worker fatigue and the use of contractors in day-to-day plant maintenance.

An agreement was reached on March 12 on national issues including pay, benefits and the fatigue and contractor issues, but strikes continued over local issues at the Whiting refinery and others.

The major sticking point at Whiting was the union's right to bargain under the contract over policies such as those that affect worker safety at the refinery.

Workers are still on strike at the refinery in Toledo, Ohio, co-owned by BP and Husky Energy (>> Husky Energy Inc.).

Negotiations also are continuing between striking workers and Marathon Petroleum Corp's (>> Marathon Petroleum Corp) Galveston Bay Refinery in Texas City, Texas.

(Reporting by Erwin Seba; Editing by Meredith Mazzilli and Matthew Lewis)

Stocks treated in this article : Husky Energy Inc., Marathon Petroleum Corp, BP plc