Air Canada flight attendants rejected a second tentative agreement they had reached with the airline, and their union has served notice that they will be ready to go on strike one minute after midnight on Thursday.

The Canadian Union of Public Employees (CUPE) announced a tentative deal in September after the federal government threatened back-to-work legislation if flight attendants walked off the job.

Just over 65 per cent of members who voted opted to reject the agreement, the union said in a news release issued late Sunday. The airline's 6,800 flight attendants will be in a legal strike position as of 12:01 a.m. Thursday.

Duncan Dee, Air Canada's executive vice president and chief operating officer, said the airline will operate on a partial schedule in the event of a strike.

"We are perplexed and disappointed that two tentative agreements negotiated in good faith with and unanimously recommended by the democratically elected representatives of our flight attendants have failed to be ratified," Dee said in a statement. "Air Canada remains hopeful that a disruption can be avoided."

According to the statement, passengers who are booked on flights from Oct. 10 to Oct. 15 can change their travel dates until Dec. 15 free of charge. That offer will be renewed every six days on a rolling window basis.

Jeff Taylor, president of CUPE's Air Canada Component, said the vote indicates union members are frustrated with the airline. The major areas of dispute between the two sides are wages, pensions, crew reset and working conditions.

Taylor cautioned the federal government against threatening or introducing another back-to-work bill in an effort to quash a strike.

"We ask the federal government, in the strongest possible terms, to respect our right to collective bargaining and not intervene unilaterally in this dispute," Taylor said.

This is the second deal the flight attendants have rejected, having voted against another agreement in August.

When the second agreement was announced in September, CUPE leadership expressed confidence that members would give it the okay.

They also said the back-to-work legislation was unnecessary, because the two sides were close to a deal. Labour Minister Lisa Raitt argued that the threat of a back-to-work bill may have expedited the deal.

Earlier this year, Raitt threatened Air Canada's customer services employees with similar legislation after they walked off the job.

In June, the government voted to legislate striking Canada Post employees back to work.

Ashley Kelahear, a spokesperson for Raitt, told The Canadian Press Sunday that the government is disappointed by the latest vote result.

"The government will be considering its options however we will be clear that a work stoppage is unacceptable in this time of fragile economy," she said in an email.