The BP refinery in Rotterdam processes around 400,000 barrels of oil annually, and is an important supplier of diesel to Northern Europe.

"If we don't receive a reaction from BP about our ultimatum and our demands, then we will proceed to action," said spokesman Jaap Bosma of the CNV union in an interview. CNV represents 440 of the 730 workers at the refinery and says it has a mandate from them to strike.

BP said it remains in discussions with the unions and is hopeful of a resolution.

"We regret that we have not yet reached an agreement and that actions may follow," the company said in an e-mailed statement.

"We are convinced that we have offered a fair, market-based proposal...Naturally, we will remain in dialogue to reach an agreement soon."

Bosma said BP's final offer for a new collective labour agreement under discussion since April had been for a 5% wage increase and a one-time bonus of 4,000 euros ($4,000).

With Dutch inflation running around 15%, workers are seeking a 6% increase, the 4,000-euro bonus, and also an equity compensation package that would amount to another 3.5% pay increase, Bosma said.

Market observers and the Dutch government have warned of possible diesel shortages after Russian crude is banned from European ports in early December.

(Reporting by Toby Sterling; Editing by Emelia Sithole-Matarise)