Air Canada's unionised flight attendants reached an agreement with the country's largest carrier on Tuesday, ending the first strike by its cabin crew in 40 years that had upended travel plans for hundreds of thousands of passengers.

The strike that lasted nearly four days had led the airline that serves about 130,000 people daily to withdraw its third quarter and full-year earnings guidance.

The carrier said it would gradually resume operations and a full restoration may require a week or more, while the union said it has completed mediation with the airline and its low-cost affiliate Air Canada Rouge.

"The Strike has ended. We have a tentative agreement we will bring forward to you," the Canadian Union of Public Employees said in a Facebook post.

Air Canada said some flights will be cancelled over the next seven to ten days until the schedule is stabilised and that customers with cancelled flights can choose between a refund, travel credit, or rebooking on another airline.

The flight attendants walked off the job on Saturday after contract talks with the carrier failed. They had sought pay for tasks such as boarding passengers. While details of the negotiations were not immediately released, the union said unpaid work was over.

The CUPE, which represents Air Canada's 10,400 flight attendants, wanted to make gains on unpaid work that go beyond recent advances secured by their counterparts at U.S. carriers such as American Airlines.

In a rare act of defiance, the union remained on strike even after the Canada Industrial Relations Board declared its action unlawful.

Their refusal to follow a federal labour board order for the flight attendants to return to work had created a three-way standoff between the company, workers and the government.

Jobs Minister Patty Hajdu had urged both sides to consider government mediation and raised pressure on Air Canada on Monday, promising to investigate allegations of unpaid work in the airline sector.

Over the past two years, unions in aerospace, construction, airline and rail sectors have pushed employers for higher pay, improved conditions and better benefits amid a tight labour market.

Air Canada's flight attendants have for months argued new contracts should include pay for work done on the ground, such as boarding passengers.

Its CEO had on Monday in a Reuters interview stopped short of offering plans to break the deadlock, while defending the airline's offer of a 38% boost to flight attendants' total compensation.

While many customers had expressed support for the flight attendants, frustration with flight cancellations was growing.

Retiree Klaus Hickman missed a flight to Toronto earlier in the week. While he rebooked on another airline, he was concerned about returning to Calgary on time for a connecting flight to Germany.

Hickman sympathised with workers demanding better pay but is worried about his own health and travel challenges.

"They want to get more money to survive. And so it is with everybody else," he said.

Canada's largest carrier normally carries 130,000 people daily and is part of the global Star Alliance of airlines.

James Numfor, 38, from Regina, Saskatchewan, had been stranded in Toronto for two nights since returning from Cameroon for his brother's funeral. Air Canada only provided one night in a hotel for his family before leaving them without further support, he said.

He had slept in the airport with his family.

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