HOUSTON (Reuters) - Hourly workers striking against LyondellBasell Industries NV's (>> LyondellBasell Industries NV) Houston refinery voted on Thursday to ratify a new four-year contract, clearing a key hurdle on the way to ending a three-month-long strike, according to a union official.

No date has been set for the more than 400 members of United Steelworkers' Local 13-227 to return to work as an agreement for re-entering the refinery has not been made with Lyondell, said Kent Farr, a committeeman with the local.

After contract ratification, workers typically return to their jobs within 10 days to two weeks.

A Lyondell spokesman did not have an immediate comment about the vote, which took place at the union's offices on Wednesday and Thursday.

Lyondell workers will continue to walk picket lines outside the refinery's entrances until the return-to-work agreement is reached.

This was the second contract proposal on which Lyondell workers have voted.

The previous proposal, which the company called its "last, best and final" offer, was overwhelmingly rejected on April 14 because the company did not include a premium pay provision that was in previous contracts. It allowed workers to receive multiple pay differentials when working extensive overtime.

The new contract does not include the premium pay provision, but does include double-time provisions, according to the union.

After the April 14 rejection, Lyondell put its final offer in place at the refinery and invited strikers to cross picket lines and return to work.

The strike at Lyondell's Houston refinery began on Feb. 1 as part of the largest work stoppage by U.S. refinery and chemical plant workers in 35 years. The strike spread to 15 plants, including 12 refineries accounting for one-fifth of national refining capacity.

In addition to Lyondell's Houston refinery, strikes are continuing at three other refineries. Workers at BP Plc's (>> BP plc) Whiting, Indiana, refinery are waiting for a return-to-work agreement prior to voting on a new contract.

Talks are ongoing at Marathon Petroleum Corp's (>> Marathon Petroleum Corp) Galveston Bay Refinery in Texas City, Texas, and the BP-Husky Energy (>> Husky Energy Inc.) joint-venture refinery in Toledo, Ohio.

Lyondell has kept the 263,776 barrels per day Houston refinery in operation with temporary replacement workers.

The new contract provides pay raises of 2.5 percent in the first year, 3 percent in the second and third years and 3.5 percent in the final year.

(Reporting by Erwin Seba, editing by G Crosse)