The Canada Industrial Relations Board (CIRB) has declared the continued strike activity from Air Canada flight attendants represented by the Canadian Union of Public Employees (CUPE) as unlawful.

The board has ordered CUPE’s leadership to instruct the flight attendants to return to work.

It also directed individual members of the union to cease all unlawful activities and return to work.

The board wants the union to release a public notice to all of its members by noon EDT on Monday, declaring it has revoked its authorization for strike activities.

The Minister of Jobs and Families, Patty Hajdu, sent the dispute to binding arbitration on Saturday and ordered operations to resume.

This led Air Canada to plan to resume operations on Sunday, but CUPE’s decision to continue striking led them to suspend that plan.

The airline now estimates that 500,000 customers’ flights have been cancelled as a result.

CUPE represents over 10,000 flight attendants with Air Canada.