WestJet airplane mechanics hit the picket lines Friday evening in a surprise move that threatens to disrupt flights for thousands of travellers at the outset of the Canada Day long weekend.

Canadians can expect “severe travel disruption” if the abrupt strike by the Aircraft Mechanics Fraternal Association (AMFA) is not called off, WestJet said.

The unexpected job action comes one day after the airline expressed relief they had averted a work stoppage thanks to a ministerial order for binding arbitration on Thursday.

The country’s second-largest airline has again asked for immediate intervention by the federal labour minister and the Canada Industrial Relations Board.

“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,”  said WestJet Airlines president Diederik Pen in a release.

The union said in a statement its 680-odd WestJet workers walked off the job at 7:30 p.m. ET., arguing that the carrier’s “unwillingness to negotiate with the union made the strike inevitable.”

However, WestJet said that because the country’s labour tribunal is undertaking arbitration to resolve the impasse, a strike gives the union no leverage and amounts to “pure retaliation.”

Earlier this month, the mechanics voted to reject a tentative deal with the Calgary-based airline, prompting WestJet to seek government intervention and resulting in two strike notices issued by the union.

With the clock ticking down toward a Friday deadline, Labour Minister Seamus O’Regan on Thursday directed the airline and the union into binding arbitration, seemingly steering clear of a work stoppage that would have upended travel for up to 250,000 passengers over the long weekend.

On Thursday night, WestJet and the union both said they would follow the order, with a work stoppage apparently off the table.

“AMFA has confirmed they will abide by the direction. Given this, a strike or lockout will not occur, and the airline will no longer proceed in cancelling flights,” WestJet said Thursday.

The reversal on Friday night therefore came as an even bigger shock.

In an update to members obtained by The Canadian Press, the union negotiating committee cited the Charter of Rights and Freedoms’ protection of collective action.

It also said the industrial relations board had not expressly barred strikes and lockouts while the tribunal undertook arbitration following O’Regan’s directive.

“Because the referral by the minister was silent on the issue, AMFA members’ constitutional right to strike must prevail,” the union committee claimed.

“Having had no indication that the board would revoke AMFA’s strike notice, AMFA directed its members to cease all work.”