Thousands of Canadians are having their long weekend travel plans upended and WestJet says it expects "significant disruption" after mechanics walked off the job Friday night despite the Minister of Labour ordering both sides to arbitration earlier this week.

The Aircraft Mechanics Fraternal Association announced on Friday evening that mechanics would begin strike action at 6:30 p.m. CDT.

Minister of Labour Seamus O'Regan had instructed the Canada Industrial Relations Board to bring the two sides together in binding arbitration on Thursday. Travellers breathed a sigh of relief with the announcement, and it seemed all would continue as normal as the July long weekend approached.

However, the union later released a statement saying that the board found "that the ministerial referral does not have the effect of suspending the right to strike or lockout." The union says that was a key point in the tribunal order, and that its members did have the right to strike.

WestJet says it is "outraged AMFA is commencing strike despite Minister’s order for arbitration and public commitment to abide by it."

“The government has officially stepped in to provide binding arbitration and ensure we get to a resolution; the only reason for this union to continue with a strike action is to create damage, disrupt the travel plans of thousands of Canadians over the July long weekend and to inflict significant costs on our business,” said Diederik Pen, President of WestJet Airlines and Group Chief Operating Officer.

“Given arbitration has been ordered, a strike has no leverage on the arbitration’s outcome, so it is pure retaliation of a disappointed union," Pen added. "We are extremely outraged at these actions and will hold AMFA 100 per cent accountable for the unnecessary stress and costs incurred as a result."

The airline says that around 20,000 people will be impacted by the first round of flight cancellations.

The union says WestJet is at fault, accusing the airline of pulling out of collective bargaining discussions once the labour minister ordered arbitration.

Beginning Saturday, June 29, WestJet will begin parking aircraft in stations across Canada with the intention of operating a significantly reduced schedule by end of day.

Travellers are encouraged to check their flight status before heading to the airport.