A historic legal bid has been launched to include the sea off the Seven Sisters coastline in the proposed South Downs national park.

The South Downs Campaign is calling on the Countryside Agency to include seabed within a national park for the first time.

Members believe the status will lead to more funding for archaeology and history projects and stiffer protection for the coast.

Developments such as wind farms would be much less likely to be approved on coastline covered by national park status.

Campaigners also hope bylaws limiting speedboats and jet skis will also be enforced.

The challenge has been backed by organisations such as the RSPB and the Campaign to Protect Rural England.

The application will be made tomorrow during the lengthy and ongoing South Downs national park public inquiry.

Campaigners want to see the national park boundary extended to cover the coast and seabed stretching from the Wish Tower in Eastbourne to the Martello Tower at Seaford.

The zone includes Seven Sisters and Beachy Head.

The application has so far been thwarted by the Countryside Agency's strict interpretation of the law.

The National Park Act passed in 1949 allows for the establishment of national parks in England. The legal definition of "England" extends only to the average low tidemark.

The agency, which allocates the area of national parks, claims marine areas cannot be included.

But it will face a challenge from the South Downs group, which insists the seabed can fall within the national park.

Chris Todd, from the campaign, said: "This is a very special stretch of coastline. This challenge is about protecting the sea and also the dramatic white cliffs which signify England for many people.

"The area is a voluntary marine conservation area because of the rich archaeology, wildlife and other geographical features.

"But the understanding of marine issues is very poor because our interpretive work has not been as good as it should be."

The 37-year-old from Hollingdean, Brighton, said national park status for the sea area should also lead to more restrictions on noisy watersports.

He added: "When walking along the South Downs your whole experience can be shattered by speedboats or jet skis whizzing up and down.

"The national park authority would be able to enforce the sort of restrictions that would protect enjoyment of the tranquillity of the park."

Mel Capper, of the Countryside Agency, said the Channel could be added to the national park following changes in the law.

She said: "We don't dispute the area of sea off Seven Sisters and Beachy Head meets the natural beauty and recreation criteria for inclusion in the national park.

"The area is designated as a Heritage Coast and a Voluntary Marine Conservation Area because of its obvious natural beauty.

"Our legal advice is that the National Parks Act does not allow us to designate a national park below the mean low water mark. To include the marine area needs new legislation."

At present there are six national parks in England which reach the coast but do not include the marine area.

Ruth Chambers, deputy chief executive of the Council for National Parks, said: "We are interested in the suggestion the South Downs national park should include marine areas. We think it will be helpful for the public inquiry to look at this issue and we will be keeping a close eye on the outcome."

Adrian Thomas, of the Sussex RSPB, said: "We are committed supporters of the South Downs Campaign.

"Potential national parks' marine extensions would see the admirable protection of small areas of the coastal environment.

"But the RSPB believes the answer for protecting the marine environment needs to be comprehensive and specific legislation and management."

The inquiry will continue to hear evidence until September.

A decision on national park status for the Downs is expected at the start of next year.