A national strike in Britain set for Wednesday threatens to paralyze operations at Europe's busiest airport and cut many public services throughout the country.
British Airport Authorities warned airlines Friday that they need to reduce service by at least 50 percent on November 30 because customs and immigration operations are expected to be greatly affected.
Norman Boivin, chief operating officer at Heathrow, advised airlines that "gridlock" is expected - with queues for immigration so long that incoming passengers would be unable to leave their planes, and aircraft unable to park at overcrowded terminals.
On a typical day, Heathrow airport receives more than 180,000 passengers.
Hospital services will be sharply curtailed and almost all schools are expected to close if the 24-hour strike goes ahead as planned.
The Trades Union Congress, an umbrella organization of British unions, expects more than two million workers to join the strike - a protest against government plans to reduce a $185 billion budget deficit by reducing pension benefits and increasing contributions by wage-earners.
At Heathrow, Boivin predicted mass cancellations of departing flights and said flights bound for Britain might have to be diverted to other destinations.
Authorities at London's second largest airport, Gatwick, also have asked airlines to sharply reduce the number of arriving passengers on Wednesday, in part by allowing travelers to change their flight bookings cost-free.
Some information for this report was provided by AP and AFP.