An alternative to the controversial expansion of Gatwick could lead to massive flooding, it was claimed today.

The damning verdict on plans to build a third runway at London Heathrow is a blow to campaigners in Sussex.

Heathrow had been considered the Government's favoured option to massively expand air capacity in the South-East.

But a report published today said the proposal could expose residents in parts of west London and Surrey to a serious flooding risk.

The document, commissioned by campaigners opposed to Heathrow's expansion, suggests massive gravel extraction to build new facilities would damage a natural "sponge" which soaks up ground water.

Heathrow is in the middle of a flood plain, where water from the Chiltern Hills flows into the Thames.

John Stewart, chairman of the pressure group Hacan ClearSkies, said: "The worst-case scenario is that if a third runway is built, there will be extra run-off water. If that's not dealt with, we could have significant flooding."

John McDonnell, the Labour MP for Hayes and Harlington, is expected to call for the Government to carry out a detailed flood risk assessment.

Hacan's study suggests the Thames, Colne and Spelthorne rivers could struggle to cope with heavy rain.

The Government announced three possible options for extending Gatwick earlier this year, including a new full-length runway to the south and parallel to the existing one, a runway about 1,000 metres to the south, or two new runways - one to the south and a new runway to the north.

It would destroy 430 homes and is fiercely opposed by residents' groups.