TORONTO — The Canada Industrial Relations Board declared a strike by 10,000 Air Canada flight attendants illegal Monday and ordered them back on the job after they ignored an earlier order to return to work and submit to arbitration.

The strike at Canada’s largest airline entered its third day on Monday and is affecting about 130,000 travelers per day during the peak summer travel season, and the two sides remain far apart on pay and other issues. Air Canada suspended plans to restart operations Sunday after the union defied an earlier return-to-work order.

“The members of the union’s bargaining unit are directed to resume the performance of their duties immediately and to refrain from engaging in unlawful strike activities,” the Canada Industrial Relations Board board, or CIRB, said in a written decision.

The board, an independent administrative tribunal that interprets and applies Canada’s labor laws, said the union needs to provide written notice to all of its members by noon Monday that they must resume their duties.

It was not immediately clear what recourse the board or the government has if the union continues to refuse.

“We are in a situation where literally hundreds of thousands of Canadians and visitors to our country are being disrupted by this action,” Prime Minister Mark Carney said. “I urge both parties to resolve this as quickly as possible.”

Carney said his jobs minister would have more to say later. Carney said it was disappointing that talks have not led to a deal, and added that it is important that flight attendants are compensated fairly at all times.

The labor board previously ordered airline staff back to work by 2 p.m. Sunday and for the union to enter arbitration, after the government intervened. Air Canada then said it planned to resume flights Sunday evening. But when the workers refused, the airline said it would resume flights Monday evening instead. However, there was no sign that the Canadian Union of Public Employees, or CUPE, would relent.

Air Canada operates around 700 flights per day.

CUPE national President Mark Hancock on Sunday had ripped up a copy of the initial back-to-work order outside Toronto’s Pearson International Airport, and said members wouldn’t go back to work this week, to the cheers of picketing flight attendants.

Flight attendants walked off the job around 1 a.m. EDT on Saturday, after turning down the airline’s request to enter into government-directed arbitration, which allows a third-party mediator to decide the terms of a new contract.

Air Canada and CUPE have been in contract talks for about eight months, but remain far apart on the issue of pay and the unpaid work that flight attendants do when planes aren’t in the air.

The airline’s latest offer included a 38% increase in total compensation, including benefits and pensions, over four years, that it said “would have made our flight attendants the best compensated in Canada.”

But the union pushed back, saying the proposed 8% raise in the first year didn’t go far enough because of inflation.

Passengers whose flights are impacted will be eligible to request a full refund on the airline’s website or mobile app, according to Air Canada.

Last year, the government forced the country’s two major railroads into arbitration with their labor union during a work stoppage. The union for the rail workers is suing, arguing the government is removing a union’s leverage in negotiations.