Petro-Canada has locked out 260 unionized employees at its Montreal refinery, the company announced Saturday.

In a statement, Dan Sorochan, Petro-Canada's vice-president of refining and supply, said management will assume operations during the lockout.

"By taking this action, we hope to move the negotiation process forward and reach a settlement," Sorochan said.

He said Petro-Canada has operated the plant in a constant state of standby due to the risk of a strike by workers with 12 hours notice.

The company and the union have been working on a settlement since December 2006. The collective agreement expired in February.

The union said negotiations have been "horrible" and denounced the lockout. Union representative Daniel Cloutier said it's unacceptable for such a large, profitable company to impose major conditions on its employees.

He said Petro-Canada wants the union to accept reduced rights and members to swallow a claw-back on their seniority rights. Cloutier also said the company wants to break up a national bargaining pattern that has existed for 40 years.