Airlines have been cancelling some flights while others, including Air Canada, have been allowing passengers to rebook their flights because of an imminent labour disruption affecting the London Heathrow Airport.

As many as two million public workers in Britain, including immigration officials, teachers and garbage collectors, are expected to join a one-day walkout on Wednesday in an escalating dispute over planned changes to public-sector pensions.

Airlines in the Middle East and Greece have already cancelled flights into Heathrow.

Many airlines, including British Airways and Virgin, have advised passengers to reschedule their trips into London on Wednesday.

Air Canada said passengers can change their flights free of charge to another date between Nov. 28 and Dec. 7 using its self-serve rebooking tool or by contacting Air Canada Reservations.

The airline also advised passengers travelling to, from or via the London-Heathrow airport to check their flight status or call its automated flight information system at (888) 422-7533.

The planned walkout is expected to top the scale of Britain's 1979 strikes when tens of thousands of people halted work over pay disputes.

In the latest dispute, workers oppose government demands that they work longer before receiving a pension, contribute more money each month and accept a pension calculated using their average career salary, rather than their final salary.

With files from The Associated Press