Air Canada cancelled 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 that said its members must 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."

Hancock ripped up a copy of the back-to-work order outside the airport's departures terminal where union members were picketing on Sunday morning as a way to signal to Air Canada that "we're ready for a big fight."

In a statement on Sunday afternoon, the airline said CUPE "illegally directed i

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