The Canada Industrial Relations Board has declared a strike by 10,000 Air Canada flight attendants illegal and ordered them back on the job after they ignored an earlier order to return to work and submit to arbitration.

The Canadian Union of Public Employees contends that the government’s decision on Saturday to end the strike by taking it to binding arbitration is itself illegal and violates its members’ constitutional right to strike. It filed an application for an injunction to block the back to work order with the Federal Court of Canada over the weekend.

Union leaders for Air Canada’s flight attendants face fines and criminal charges after defying a second order from Canada’s federal labor board to end their “unlawful” strike.