Air Canada's striking flight attendants on Sunday refused a government-backed labour board's order to return to work, forcing the airline to delay restarting its operations and leaving its passengers in limbo.

The Canadian Union of Public Employee said the 10,000 Air Canada attendants it represents would remain on strike, calling the order unconstitutional and "designed to protect the airline's profit."

Instead, it invited Air Canada - the country's largest airline - back to the table to "negotiate a fair deal."

In response, the airline said it would delay plans to restart operations from Sunday until Monday evening.

The refusal by the union to obey the order left many travellers at Toronto Pearson International Airport confused and frustrated on Sunday afternoon.

Many of them were camped out in airport lounges, uncertain whether when and if flights would resume or whether Air Canada would make tentative arrangements.

"We are kind of left to figure it out for ourselves and fend for ourselves with no recourse or options provided by Air Canada at this time," said Elizabeth Fourney of Vancouver.

Francesca Tondini, a 50-year-old from Italy, said she was about to return home after visiting Canada when her flight was cancelled on Saturday and again on Sunday.

When she asked Air Canada when the flight would finally depart, the airline responded, "maybe tomorrow, maybe Tuesday, maybe Friday, maybe Saturday - they don't know!," she said.

The flight attendants began their strike early on Saturday morning, after negotiations that had dragged on for months reached an impasse.

In anticipation, the airline canceled most of its 700 daily flights, forcing more than 100,000 travelers to scramble for alternatives.

Within hours of the strike declaration, the Canada Industrial Relations Board complied with a request by Jobs Minister Patty Hajdu and ordered binding arbitration.

The Canada Labour Code gives the government the power to ask the CIRB to impose such an order in the interest of protecting the economy.

Air Canada had encouraged the government to act, while CUPE had pushed for a negotiated solution, saying binding arbitration would take pressure off the airline.

It is exceedingly rare for a union to defy a back-to-work order. In 1978, Canadian postal workers refused to comply with back-to-work legislation, resulting in fines and the jailing of their union leader for contempt of Parliament.

The government's best option is to go to court to enforce the order and secure a contempt order if the union refused to back down, said Michael Lynk, professor emeritus at Western University's Faculty of Law in London, Ontario.

"The union leadership could face the same consequences as what happened 45 years ago. It could be fines against the union... potential of jail time for the union leaders," he said. (Reuters)