The Airports Commission had estimated that surface infrastructure changes would cost £5bn, but TfL responded by saying the figure underestimated the cost and required changes would cost in the realms of £15m-£20m.
A TfL spokesperson reiterated the prediction after Chris Grayling said he did not see where the £15bn could have come from.
"Expansion at Heathrow will significantly increase demand for access to the airport," they said.
Our expert analysis indicates approximately £15bn more investment will be needed beyond what is already committed and the key component of this is a new southern rail link from Waterloo to Heathrow.
Thus far, the government has given no commitments to deliver this new rail link, despite the Airport’s Commissions recommendation to do so and, without such a commitment, the aspirations for no increase in road traffic are not credible.
Transport for London analysis of Heathrow expansion surface access costs
M4 SMART Motorway
Expansion enabling works
M4 widening Junctions 2-4a
Maintenance for new roads/tunnels (to 2050)
Local road capacity and bus enhancement measures
Area traffic management
Piccadilly line Upgrade
Western Rail Access
Southern Rail Access
South West Main Line Link
Grayling made the comments yesterday while giving evidence to the Environmental Audit Committee on Heathrow's third runway.
On TfL's cost estimate for surface infrastructure changes, the transport secretary said it looked to him as if "somebody has taken every possible transport improvement in the whole of metropolitan London and thrown it into the mix, and probably funding a large chunk of Crossrail 2 out of it as well".