The union representing workers at the Co-op Refinery in Regina have served a 48-hour strike notice.

Unifor Local 594 served the notice at 4:30 p.m. CST on Tuesday, meaning workers could walk off the job as early as Thursday. The union said the dispute over choice and protection of pension plans was not resolved at the bargaining table.

In a statement, the Co-op Refinery Complex said it was disappointed by the breakdown and issued a 48-hour lockout notice less than an hour after the strike notice was served. The refinery lockout is set to start at 5:30 p.m. Thursday.

"A 48-hour strike notice creates an unsafe operating environment for the refinery," Gil Le Dessay, vice-president of operations at the Co-op Refinery, said in a statement.

"It is vital to the safety of our operation that we control the timeline of labour action as our highly-skilled management team assumes control of the Refinery's operation," Le Dessay said.

Kevin Bittman, the president of Unifor Local 594, said the only safe way to run the refinery is with Unifor workers. In a statement Unifor said it is concerned about a small group of "underqualified replacement workers" operating the refinery.

"If we are not inside, the only safe option is shutting it down. Our formal notice of job action is accompanied by an offer to shut down the units safely," Bittman said in a release.

The strike notice came after workers at the Co-op refinery voted 97.3 per cent in favour of striking, if necessary.

Mediation ended on Nov. 12 without an agreement.