Air Canada flight attendants remained on strike Sunday, defying a government labor board’s order to return to work and forcing Canada’s largest airline to delay its restart plans.

Air Canada flight attendants said on Sunday (August 17) they will remain on strike and spurn a government return-to-work order they called unconstitutional that would send them back to their duties.

The union said no, setting up an almost unprecedented standoff with the Canadian government, which had requested the back-to-work order.