Air Canada flight attendants said on Sunday they will remain on strike and challenge a return-to-work order they called unconstitutional, defying a government decision to force them back to their duties by 2 p.m. ET (1800 GMT).

Air Canada had said it planned to resume flights on Sunday evening, a day after the Canadian government issued a directive to end a cabin crew strike that caused the suspension of around 700 daily flights, stranding more than 100,000 passengers.

The Canadian Union of Public Employees said in a statement that members would remain on strike and invited Air Canada back to the table to “negotiate a fair deal.”