Passengers at Heathrow could face delays of up to 12 hours when immigration officers strike next Wednesday.
Thousands of border agency workers are expected to join the estimated 2.6 million public sector workers in next week's strike over changes to pensions.
In a letter to airlines reported in the Daily Telegraph, the airport's chief operating officer Norman Bolvin warns of a real danger of "gridlock".
"The delays in immigration are likely to be so long that passengers could not safely be 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 parking stands, mass cancellations for departing aircraft and diversions outside the UK for arriving aircraft.
"There are likely to be very long delays of up to 12 hours to arriving passengers."
Heathrow handles more passengers than any other airport in the UK.
According to the Telegraph, Virgin Atlantic and British Airways have said they would allow passengers to change their flights free of charge to enable them to avoid the expected chasos.
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