The marshes are home to more than 200,000 birds

The Royal Society for the Protection of Birds (RSPB) has asked its 1.3m members across Britain to join the protest.

The charity hopes their voice might influence the government's decision, expected later this year, on how to cope with the expected surge in demand for air travel over the next 30 years.

One proposal is to build an airport big enough for five runways at Cliffe, on the Thames estuary in North Kent.

This area really is natural, national and international heritage Perry Haines, RSPB

Others include three new runways at Stansted and a third runway at Heathrow, as well as the possible expansion of Gatwick.

The proposed site at Cliffe is being opposed by environmentalists because it is a breeding ground for birds.

Among more than 200,000 birds to make their home in its marshes are redshank, curlew, little egrets, dunlin and oyster catchers.

The avocet, the bird used by the RSPB as its symbol, also finds refuge at Cliffe.

The avocet is the symbol of the RSPB

The area is protected by a European Union directive - but campaigners fear even that is not enough to save it from development.

Perry Haines, of the RSPB, said: "This area really is natural, national and international heritage.

"If they could do that here then nowhere is safe."

Keen walker Gill Moore is one of many local residents to oppose the airport plan.

She said: "I walk quite a lot out over the marshes - I just love it.

"I think it's a wonderful, magical place and the fact that the government can even contemplate building an airport here is absolutely horrendous."