A labor relations board declared a strike by Air Canada’s 10,000 flight attendants illegal on Monday and ordered their union to tell them to return to work by midday.

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 strike stranded more than 100,000 travelers around the world during the peak summer travel season.