Airline WestJet says it’s had to cancel at least 150 flights starting Saturday after the union maintaining the airline’s planes announced it went on strike Friday night.

According to the Aircraft Mechanics Fraternal Association (AMFA), its members began strike action around 7:30 p.m. ET.

The AMFA says the move was due to the airline’s “unwillingness to negotiate with the union.”

The strike, which WestJet says is impacting around 20,000 travelers, came after the federal government issued a ministerial order for binding arbitration on Thursday, and follows two weeks of heated discussions with the union on a new deal.

Calgary-based WestJet, Canada’s second-largest airline, has decried the move by the mechanics union, saying it’s “extremely outraged at these actions and will hold AMFA 100% accountable for the unnecessary stress and costs incurred as a result.”

In an update to its members, the union’s negotiating committee referenced an order by the Canada Industrial Relations Board that does not explicitly bar any strikes or lockouts, as the tribunal undertakes arbitration following Labour Minister Seamus O’Regan’s directive on Wednesday.

WestJet says Canadians can expect “severe travel disruption” if the abrupt strike is not called off.

Those who have booked with WestJet are being advised to check the status of their flight before heading to the airport.