The union for 10,000 striking Air Canada flight attendants said Monday they won’t return to work even though the strike, now in its third day, has been declared illegal.

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.