The refinery was shut last week due to technical problems.

"We regret the unions have decided to proceed," BP said in a statement, referring to the unions' warning on Monday that they would not cooperate with a restart unless BP changed its position on pay.

"As a result, the planned restart has been put on hold and operations at the refinery remain shut down," BP said.

A spokesperson declined to comment on how long the closure might last. Unions are seeking a package representing a 9.5% pay increase, while BP has offered a 5% increase and a one-time bonus.

"We do remain in dialogue with the unions to reach an agreement and minimise the consequences," BP said.

The unions will decide on what action to take next if BP does not accept their demands, or make a new offer, by 12 p.m (1100 GMT) on Wednesday.

(Reporting by Toby Sterling; Editing by Mark Potter)