Airlines are offering free re-booking of flights to London on Wednesday to spare passengers from the expected long delay in customs clearance at the city's airports due to participation of customs officers in a one-day general strike.

Qantas Airlines (QAN: AU) said passengers can rebook on alternative flights between Nov. 28 and Dec, 7 or re-route their flights without charges. British Airways (BAY:LSE), Cathay Pacific Airways (293:HK) and Virgin Atlantic Airways are offering the same free service.

The airlines are making the flight switch upon the advice of Heathrow and Gatwick airports officials, who warned of 12-hour delays in passport checks.

Normand Boivin, chief operating officer of Heathrow, also asked airlines to load only half of their planes to minimise the expected gridlock when passengers are forced to stay aboard planes because of the long queue inside the terminals.

"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 30 November to 50 per cent of normal levels," Boivin said, according to Sydney Morning Herald.

Cathay Pacific Airways also advised passengers through its website not to travel to Heathrow. UAE's Etihad Airways and Greece's Aegean Airlines have cancelled flights to and from Heathrow.

UK Border Agency staff, teachers, health workers and civil servants will be among the expected two million workers walking out of their jobs Wednesday in disagreement to plans by the government to extend retirement age so they can contribute to the pension fund longer. Prime Minister David Cameron said the move will save the government $126 billion.

The strike also could affect flights in and out of the London area's other three airports - Stansted, Luton and City - which handle many flights to and from continental Europe, according to the Associated Press.