An ongoing labour dispute between the District of West Vancouver and the union representing its bus drivers could cause disruptions for thousands of transit riders as early as Monday.

The drivers aren’t going on strike - yet - but they’re planning to stop working overtime, an action spokesman Bill Tieleman said could escalate into a full work stoppage by week’s end.

Tieleman said he’s “mystified” by the fact that the district and the Amalgamated Transit Union have been unable to reach an agreement. The union has been out of contract in West Vancouver since March, despite the fact that it has reached new agreements with other jurisdictions.

“They're asking for concessions in benefits, and they're also not really being willing to try to maintain mechanics,” Tieleman said. “There’s 14 mechanics, and there have been 14 vacancies or changeovers in the last two years alone ... Almost every single union and every single employer has settled, except West Vancouver.”

For his part, West Vancouver Mayor Michael Smith seemed equally mystified by the lack of an agreement.

He told CTV News the district has not demanded any concessions, and has a long history of providing wages and benefits on par with those agreed to elsewhere in the region.

Smith said the ATU is the only union he’s ever had trouble working with as mayor.

“I think we have half a dozen unions in West Vancouver, and we don't have a problem negotiating with these other unions,” he said. “So it makes you stop and think, why do we have an issue with the bus drivers?”

The union has 149 members operating 52 West Vancouver buses, Tieleman said. They’re responsible for roughly 18,000 passenger trips per day.

He said the union doesn’t want to inconvenience those riders “if at all possible,” but that even stopping overtime could cause disruptions if something unexpected - such as a bus breaking down at the end of a mechanic’s shift - happens.

With files from CTV Vancouver’s Sarah MacDonald