Transport for London (TfL) has raised "serious concerns" about congestion and the costs of expansion at Heathrow just weeks before the Airports Commission's final recommendation is due.

TfL said both the airport and Commission had "significantly underestimated" the challenge of improving transport access to the site, with the Airports Commission estimating £5bn would be enough to make the improvements.

But TfL believes to provide an optimal level of service, the figure would be nearer to £20bn, raising questions about who would pay the additional costs.

The commission said building a third runway and its related assets would cost £18.5bn, excluding surface access.

A population growth of 37 per cent by 2050 has also not been taken into account in regards to the increased pressure on London's roads and public transport infrastructure, TfL said.

The Airports Commission acknowledges that if the west London airport did expand, passengers would experience "very crowded conditions during peak times", but TfL said it had underestimated the demand for surface access in 2030 by 25 per cent.

Daniel Moylan, Mayor of London's chief adviser of aviation, said: "The Airports Commission’s assessment of the surface access impacts for an expanded Heathrow does not extend beyond 2030 and assumes a third runway to be barely a quarter full, which is not credible for an airport currently operating at over 98 per cent capacity.

"With that in mind it is hardly surprising that the commission has woefully underestimated the associated surface access cost by more than £10bn."

TfL's concerns were aired in a letter to the All Party Parliamentary Group (APPG) for Heathrow and the wider economy.

Zac Goldsmith, Conservative prospective parliamentary for Richmond Park and north Kingston, MP and chairman of the APPG, said: "TfL is better placed than any other organisation to understand the effects Heathrow expansion will have on London’s transport network, and it is extraordinary therefore that the commission never bothered to ask for its assessment.

"This raises serious questions about the thoroughness and reliability of the commission’s work. If TfL is right, the taxpayer may end up having to cough up an additional £15bn to help Heathrow secure its monopoly, in addition to all the associated problems of gridlock, noise and air pollution."

To read TfL's response in full, visit heathrowappg.com/tfl-response-to-appg-on-surface-access.