Campaigners have begun putting their case for more land to be included in a proposed South Downs national park to fulfil a 50-year-old ambition.

The South Downs Campaign said it supported the Countryside Agency's proposal for the park, but insisted more land could be added in many places.

Chairman Robin Crane told a public inquiry the plan had been the dream of conservation groups for more than half a century.

He said: "In the view of many the South Downs is the most iconic of English lowland landscapes.

"The hills that form its rippling backbone are the best-loved of all chalkland landscapes."

He said boundaries were notoriously difficult to alter once a national park had been established, and strongly supported including parts of the Low Weald as well as the more famous chalk hills.

Opponents of a national park have suggested only the chalk hills of Sussex and Hampshire should be included, contrary to the Countryside Agency's proposal.

Mr Crane said the geology and biodiversity of the whole area under consideration was of national and international importance, and the heaths of the Low Weald were as important as chalkland.

He said many pockets of land on the fringes of Brighton and Hove should be included, as well as cliffs between Brighton and Newhaven.

The ruling national park authority should also be given more power to control marine areas off Beachy Head and the Seven Sisters.

He said Lewes and Arundel should both be in the park, as the Countryside Agency proposes, and the boundary should be extended in the Adur, Ouse and Cuckmere valleys.

The hearing is due to run until September.

A Government decision is not expected until 2005.