Campaigners say the areas off the coast deserve to be protected

They have told a public inquiry the areas deserve to be protected as they are of archaeological and geological interest, and rich in marine wildlife.

The Countryside Agency has said they cannot be included because they are not "land" and are therefore not eligible.

A decision on the proposed national park is due to be made in 2005 or 2006.

Shipwreck souvenirs

Campaigners believe gravel extraction, the dumping of partially-treated sewage and treasure hunters seeking souvenirs from shipwrecks all pose a threat.

If they succeed in having the marine areas off the coast included, they would form the UK's first maritime national park.

Paul Millmore of South Downs Campaign said it would be "environmental philistinism" to exclude the area beyond the shoreline just because it is not considered "land".

"It is fascinating in terms of its geology, there is a mass of marine archaeology under the water - on very low tides you can walk out to the shipwrecks - and it is also full of fascinating marine wildlife," he said.

David Thompson of the Countryside Agency said: "National Parks, under the legislation that allows them to be created, can only include areas of England, and the area beyond the low-water mark is, technically, outside of England and so cannot be included."

The proposed South Downs National Park includes 73 miles of chalk hills and extends across more than 400 acres, from Eastbourne in East Sussex to Winchester in Hampshire.