New submissions to the UK’s Airports Commission from the major air alliances support Heathrow's arguments for building a new runway at the hub.
London’s main airport is seeking approval to expand in competition both with other London gateways and with schemes to build a new airport from scratch.
The Airports Commission, chaired by Sir Howard Davies, is due to report in 2015 on how the UK should maintain its air connectivity.
But a letter to the commission from Tae Joon Kim, SkyTeam’s VP of airport services, said the alliance’s carriers would not consider moving to another London airport.
"Relocating to another London or UK airport is not an option for our members," he said.
Oneworld, led by British Airways, has affirmed that it wants to stay at Heathrow, while Star is due to move into the gateway’s Terminal 2 next year.
The alliances' united front is a challenge for Gatwick's expansion plans, as the rival gateway has made a case that it could host one of the three global air alliances.
A London-based airline insider said the alliances’ unwillingness to be separated makes strong commercial sense.
"Any alliance moving out would be handing all its premium business to its competitors," said the expert.
"I don’t think the government could force any of the alliances to move and I can’t see that any of them would move voluntarily."
In the source’s view, an alliance moving out of Heathrow on its own would be committing "commercial suicide".
The expert also stressed that alliances are especially keen to share a gateway with the national carrier, British Airways.
"If they moved, they would suddenly lose connections with the domestic market. While easyJet flies a lot of domestic flights at other London airports, it doesn’t interline with carriers for international routes."
In theory, all three main alliances could move together to a new airport with hub capacity.
London’s mayor, Boris Johnson, who opposes expansion at Heathrow, has backed a proposal for a new four-runway 'super-hub'.
But the cost of closing Heathrow and building a new airport is widely seen as prohibitive.