Sadiq Khan has welcomed the potential for direct train services from Waterloo to Heathrow Airport but warned that there may not be enough capacity on the rail network for such a service to be introduced.
Labour London Assembly member Fiona Twycross tabled a question to Mayor of London Sadiq Khan asking whether he backed calls by Seema Malhotra MP for a rail link between Heathrow Airport and Waterloo Station.
Mr Khan's reply was published this week.
The Mayor said: "While the potential for a new connection between Heathrow and Waterloo is welcome, the proposals face a serious capacity challenge.
"Rail lines between Windsor and Waterloo are severely constrained and the multiple level crossings on the route limit the ability to accommodate additional trains.
"Any new airport service cannot be at the expense of existing and planned services or the network's ability to meet forecast growth in background demand.
"If the Government is to take forward a third runway at Heathrow airport, it needs to demonstrate that there is both the rail connectivity and capacity to enable expansion and achieve the airport's stated aspiration of a zero increase in passenger and staff highway trips.
"The Southern Rail Access proposals, reliant on the rail lines between Windsor and Waterloo, cannot provide the capacity to support an expanded Heathrow.
"There are alternatives which could deliver the capacity and connectivity required – including direct links to Waterloo – and these are being investigated by TfL.
"While the Airports Commission identified Southern Rail Access as the only rail scheme required for Heathrow expansion, it emerged last month that the Government now deems no new rail infrastructure essential for an expanded Heathrow.
"Such an approach is deeply concerning and risks worsening congestion on the roads and a further deterioration of air quality around Heathrow."