Union leaders for Air Canada’s flight attendants face fines and criminal charges after defying a second order from Canada’s federal labor board to end their “unlawful” strike.

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.

"The government will be very reticent to be too heavy-handed because in Canada the Supreme Court has ruled that governments have to be very careful when they take away the right to strike, even for p...