Airlines are cutting flights into London's largest airport on Wednesday because of fears of long delays and overcrowding when border staff join a mass strike over public sector pensions.

Airports operator BAA, owned by Spanish infrastructure group Ferrovial, has warned of delays of up to 12 hours for passengers arriving at London Heathrow, Europe's busiest airport. It has asked airlines to halve the number of international passengers flying into Britain on Wednesday.

Around two million public sector workers, including UK Border Agency (UKBA) staff, are set to walk out on Wednesday over reforms that will make them work longer and pay more for their pensions, part of a raft of austerity measures imposed by the Conservative-led coalition aimed at cutting Britain's budget deficit.

Abu Dhabi's Etihad Airways on Monday said it had cancelled three flights to London's Heathrow airport on Wednesday, November 30. and reduced capacity on another service, while Dubai's Emirates said it may be forced to cancel some services.

Heathrow's largest carrier British Airways, and Britain's Virgin Atlantic and bmi have encouraged customers not to fly into Britain on November 30 and said they would allow passengers to switch flights to a different date free of charge.

This is another serious kick in the teeth for UK Plc -- Britain cannot afford to be closed for business, a Virgin Atlantic spokesman said.

Along with airport operators, wider aviation industry and other airlines, we are in continued discussions with government and UKBA about possible contingencies.

Passengers arriving on international flights into London or transferring through London onto a domestic flight will be hit, but those departing the capital or travelling on UK domestic flights will be unaffected.

The British government is flying home embassy staff and training volunteers from other departments to reinforce during the strike and help ensure passports are checked as quickly as possible at ports and airports.

London's second largest airport, Gatwick, has also asked airlines to rebook passengers.

The UKBA has said it expects to perform at less than 50 percent productivity on Wednesday.

(Editing by Helen Massy-Beresford)