LAND on Cliffe Marshes, already threatened by plans for an airport, is being bought up in connection with a port scheme. Conservationists and landowners are baffled and alarmed.

They have discovered that the Port of London Authority has been served with a compulsory purchase order to buy marsh land it owns. The order is part of the plan to build a new container port on the Essex side of the Thames to replace an old oil refinery.

The developers, the Peninsular and Oriental Steam Navigation Company (POSN), say they have to buy the land at Cliffe to replace land they will destroy on the Essex shore. But the proposals have led to confusion and concern in the wake of rumours that a four-runway London airport is to be built at Cliffe.

Robin Theobald, from the Dickens' Country Protection Society, said the land is already covered by various conservation orders. He said: "It is very suspicious. It does not make sense. They don't need to compulsorily purchase it because it is already being managed for conservation."

Former councillor and Cliffe resident Chris Fribbins said: "The fact that land is being bought up for environmental reasons does not make me feel any safer about the plans for an airport. The airport proposals may well override anything that goes before it."

Landowner Ron Lamb has his own worries. He lives at Wateringbury but owns 1,400 acres of land next to the area being compulsorily purchased. He says he has been kept completely in the dark about the developments.

"Whatever they do we will be the most affected," he said. "We are very concerned about any development. If they start taking out lots of sediment from the Thames in front of us it would affect our land. The sea could come through the sea wall. They would need to make a new sea wall otherwise the water will end up coming right up to Cliffe Village."

POSN has applied to the government for a Harbour Empowerment Order to develop the land at Cliffe Marshes for environmental and ecological purposes. It includes compulsory purchase powers. The company is building a world class deep-sea port on the opposite side of the Thames. In return the company says it must buy similar land on the Cliffe side and protect it. A spokesman for POSN said: "If there are any environmental impacts in carrying out these developments we must introduce measures to address them."

Colin Duncanson, who lives near Maidstone, leases the land that is being compulsory purchased from the Port of London Authority. His family has worked it for more than 90 years. He grazes more than 1,000 breeding ewes on it and would have to re-home them. He said: "We are against giving up the land. We will put up a fight. It will fly in the face of millions of nature lovers."

English Nature, a nature conservation advisor, has been approached by P&O about the plans but no formal consultation has started. They will be asked for their views by Stephen Byers, the Secretary of State for Transport, Local Government and the Regions.

At the beginning of March it was rumoured Transport Secretary Stephen Byers was considering plans to build an airport on the marshes. The Royal Society for the Protection of Birds, which owns land at Cliffe Marshes, home to more than 70,000 birds, was "totally opposed to the idea".