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 presidents of two major Quebec unions fear workers in the province could soon face the same treatment as striking Air Canada flight attendants under a provincial law passed in May.

The federal government will launch a probe into allegations of unpaid work in the airline sector that Air Canada flight attendants are citing as one of the key reasons for their strike, Jobs Minister...