The transport authority has raised concerns around the Airports Commission's (AC) Final Report and its impact of a third runway at Heathrow on London's road, rail and Tube networks.
It is understood it would cost between £15bn to £20bn to improve the transport infrastructure in order to transport passengers.
Speaking to the London Assembly Transport Committee, Richard De Cani, Director of Strategy and Policy at TfL, said: "The simple word would be congestion - congestion on the road network, congestion on the rail network of a scale that we haven't seen.
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"The level of crowding you would have on those rail corridors into central London would be some of the worst that we currently see in London and that's based on 2030 demand, the year of opening.
"So it's a level of crowding and congestion that we believe would start to impact quite significantly on the whole performance of the transport network across west and south west London."
Chair of the Committee, Valerie Shawcross CBE AM, said: "Today we heard that there are clear discrepancies between the Airports Commission's assessment of transport demand with TfL's own analysis.
"For example, the Airports Commission has only planned for passenger demand up to 2030, and does not take into account transport needs with the airport at full capacity after expansion.
"Before the Government makes its decision on airport expansion, there are big questions to be answered around what transport infrastructure is needed and who will pay for it. We can’t allow a bigger Heathrow to clog up London’s roads and public transport network."
(LM/CD)
