Gatwick has a slight edge over Heathrow in betting over who will get a new runway.
Both airports are 5/6 on with bookmaker Betway to get the go-ahead for a new landing strip.
But Betfair has Gatwick as 4/7 on favourite, with Heathrow 5/4 for a third runway. That means anyone staking £100 on Heathrow getting a third runway would get £225 back should they be proved right.
The Airports Commission is considering the rival expansion bids and is expected to make its final recommendation shortly, following the completion of a consultation on air quality. The Government must then decide whether to go with the commission's findings.
Support for a third runway at Heathrow could prove awkward for the Conservative government, with several influential Tory MPs opposed to expansion at the airport.
They include London Mayor and Uxbridge & South Ruislip MP Boris Johnson, cabinet minister Justine Greening, and Richmond Park MP Zac Goldsmith, who has promised to trigger a by-election if Heathrow is chosen.
However, there are also a number of Kent, Sussex and Surrey MPs ready to fight against a second runway at Gatwick, and their opposition could prove tricky for Prime Minister David Cameron given his Conservative government has such a small majority.
Betway's Alan Alger said: "Increasing UK airport capacity is broadly supported by most MPs, but the devil is in the detail and all hell could break loose depending on the recommendation of the Davies [Sir Howard Davies, chairman of the Airports Commission] report.
"David Cameron's honeymoon period with his new Commons majority may be over before it even started. The decision to hand a new runway to either Heathrow or Gatwick could split his front and back benches, and see the Tories fighting a by-election before George Osborne has even had time to deliver his first budget."
