A labor relations board declared a strike by Air Canada’s 10,000 flight attendants illegal on Monday and ordered their union to tell them to return to work by midday.

Air Canada has suspended its plan to resume flights and unionized attendants continued picketing on Monday, despite a federal back-to-work order.

The Canadian Union of Public Employees said it will challenge an order by the Canada Industrial Relations Board to return to work by 2 p.m. ET.