Airlines have been asked to halve the number of international passengers flying into London's largest airport next 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, on Friday warned of delays of up to 12 hours for passengers arriving at Heathrow.

The 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.

Two million public sector workers could 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 the budget deficit.

We will plan for a normal flight schedule, but we are requesting all carriers to reduce load factors on each international flight arriving into Heathrow on November 30 to 50 percent of normal levels, Normand Boivin, Heathrow's chief operating officer said in a letter sent to major airlines.

The delays at immigration are likely to be so long that passengers could not be safely accommodated within the terminals and would need to be held on arriving aircraft.

This in turn would quickly create gridlock at the airport with no available aircraft parking stands, mass cancellations of departing aircraft and diversions outside the UK for arriving aircraft.

British Airways, Heathrow's largest carrier, and Virgin Atlantic are offering alternative dates to passengers due to travel on November 30.

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

The UK Border Agency (UKBA) currently expects to perform at less than 50 percent productivity. We have reluctantly concluded that UKBA will be unable to provide a contingency plan to support normal operations, added Boivin.

(Reporting by Rhys Jones Editing by James Davey)