Air Canada flight attendants have vowed to stay on strike despite being ordered back to work by the country’s labor board, deepening travel chaos for thousands of passengers.

The Canadian Union of Public Employees (CUPE), which represents more than 10,000 flight attendants, said on Sunday that its members would remain off the job and urged Air Canada to return to the table to “negotiate a fair deal.”

The defiance comes after the Canada Industrial Relations Board (CIRB) directed the workers to end their strike by 2 p.m. ET (18:00 GMT), an order the union has dismissed as unconstitutional.

The strike, which began on Saturday, is the first by Air Canada flight attendants since 1985. It has already disrupted travel for about 130,000 people each day during the peak summer season, with long delays and cancellations hitting passengers across the world. Air Canada said it expects flights to resume gradually on Monday evening.

The Canadian government moved swiftly over the weekend to try and end the standoff, asking the CIRB to impose binding arbitration under the Canada Labour Code a law that gives Ottawa power to step in when a strike threatens the wider economy. While Air Canada welcomed the decision, union leaders said it undermines their right to bargain.

“It is unusual for a union to defy a CIRB order,” analysts noted, adding that it is unclear what steps the government could take if the strike continues.

Union officials say the core of the dispute is pay fairness. Flight attendants are demanding compensation for time spent on the ground between flights and while helping passengers board, noting that they are only paid once planes are in motion.

Workers also argue that the airline’s proposed wage increases are too small to match inflation or even keep up with the federal minimum wage.

In Toronto on Sunday, other labor unions joined the picket lines in solidarity. “They are here because they see our rights being eroded,” said Natasha Stea, a flight attendant and local union president.

The dispute has put Canada’s largest airline under mounting pressure to strike a deal quickly, as passengers brace for continued disruption in the busy travel season.