A third runway at Heathrow will cripple the tube, according to councillors.

Anti-expansion campaign group 2M, which includes local authorities, says that additional capacity at the airport will swamp the Piccadilly Line with tourists, causing transport misery for workers.

"Many residents already struggle to find a space, let alone a seat, when the Piccadilly Line trains roll into the borough," said Cllr Nicholas Botterill, deputy leader of Hammersmith and Fulham Council.

"If a third runway is built at Heathrow, there will be even greater numbers of tourists commuting from the airport into central London."

Transport for London (TfL) estimates that passenger numbers using the Heathrow-to-Cockfosters line will double if a third runway is built.

John Stewart, chairman of campaign group HACAN ClearSkies, said: "The doubling of passengers on the Piccadilly Line doesn't bear thinking about. HACAN is not against Heathrow Airport in principle, but we are saying enough is enough. This is a prime example of a damaging by-product of uncontrolled airport expansion."

Councillors have warned tube bosses that the airport line, which already operates at 97 per cent capacity, can only survive Heathrow expansion if a massive cash injection is made.

A spokeswoman for TfL said: "Research referred to by Hammersmith and Fulham was to investigate the impact of Terminal 5. Without concrete research into the numbers of people that would use public transport to get to Heathrow if a third runway was built, it is very difficult to predict the exact number of passengers."

Transport Secretary Ruth Kelly is expected to making a ruling on Heathrow expansion before the end of the year.