“We will be challenging this blatantly unconstitutional order that violates the Charter rights of 10,000 flight attendants,” the Canadian Union of Public Employees said in a statement.

The union representing Air Canada's striking flight attendants on Sunday said it will remain on strike and spurn a government return-to-work order that they called unconstitutional.

Air Canada, whose collective agreement with the flight attendants expired in March, said Sunday the Canada Industrial Relations Board had directed the carrier to resume operations and have the striki...