Air Canada says it is cancelling hundreds of additional flights on Sunday after the union representing its flight attendants announced the workers would remain on strike in defiance of a 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.

"Our members are not going back to work," CUPE National president Mark Hancock said outside Toronto's Pearson Airport. "We are saying no."

Union members were picketing outside the airport's departures terminal on Sunday morning.

In a statement on Sunday afternoon, the airline said CUPE "illegally directed its flight attendant members to defy a direction" from the industrial relations board.

As a result, Air Canada said it will plan to resum

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