The cancellation affects 20,000 passengers beginning Saturday.

Canada’s second largest airline, WestJet, says it has cancelled at least 150 flights affecting 20,000 passengers beginning Saturday after the union of maintenance workers announced it has gone on strike.

The Aircraft Mechanics Fraternal Association said its members started to strike Friday evening because the airline’s “unwillingness to negotiate with the union" made it inevitable.

The strike came after the federal government issued a ministerial order for binding arbitration on Thursday. That followed two weeks of turbulent discussions with the union on a new deal.

On Thursday, WestJet said the union “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.”

The surprise strike affecting international and domestic flights is happening during the Canada Day long weekend.

Calgary, Alberta-based WestJet said it was “extremely outraged” and will hold the union 100% accountable for the unnecessary stress and costs.

In an update to its membership, the union 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.