Heathrow Airport would face a black hole in excess of £16bn to pay for enhancements to road and rail links to the airport should the third runway be given the green light, Transport for London (TfL) has warned.
The total cost for the upgrades would stand at £18.4bn, TfL said, far more than the £2.2bn Heathrow estimated.
The cost will be closely scrutinised over the coming weeks as the government is expected to move towards a long-awaited decision on where to expand airport capacity in the south east.
Last year business groups had hit out at government inaction on the decision after the independent Airports Commission recommended building a third runway at Heathrow.
In the Airports Commission report, which was published last summer, it was estimated that the cost to enhance rail and road would be £5.7bn, spent on projects including putting part of the M25 in a tunnel.
But TfL warned that if Heathrow expansion is allowed to proceed without the necessary scale of infrastructure intervention and road user charging, there will be significant consequences, both for the airport and the wider region.
It is anticipated that a third runway at Heathrow would lead to an extra 30m passengers on London's transport network by 2030.
Other projects, such as alterations to the South West Main Line and Great Western Main Line, as well as enhancements of key bus corridors, should also be included in the cost, TfL said.
And TfL added that the commission had underestimated the number of extra passenger journeys on the transport network, while it failed to factor in freight.
But John Holland-Kaye, Heathrow's chief executive, said that the TfL estimate is just a "long list of anything that might be needed across London", the Financial Times reported.
The £18.4bn figure breakdown was revealed after a freedom of information request by Greenpeace.
John Sauven, executive director of the environmental group, said that the figures showed a "gaping hole" in financing for Heathrow expansion.
Earlier this year Heathrow hired four contractors to work on plans to build a third runway, despite a lack of approval from the government.