The union representing about 420 workers at the Molson Coors brewing plant on the South Shore of Montreal are on strike.

Teamsters Canada says nearly 320 workers voted 99 per cent on Sunday against the company's last contract offer and gave the union a strike mandate.

The union says the two parties do not agree on salary increases and on the establishment of a new layoff system that would not take seniority into account.

Local 1999 president Eric Picotte says working conditions have been deteriorating since Molson's merger with U.S.-based Coors in 2005 and worsened following a merger with Miller a few years ago.

Molson Coors spokesman Frederic Bourgeois-Leblanc says it is disappointed that employees walked off the job before it made a final offer because it is offering a "more than competitive wage and has a history of providing well-paying jobs" in Quebec.

Bourgeois-Leblanc adds that the company has turned to an unspecified contingency plan as it works to meet consumer demand.