A national park in the South Downs would give people the impression farmland had been nationalised and they could walk anywhere, a public inquiry was told.

The National Farmers' Union (NFU) said there had already been similar problems in established national parks, such as the Peak District and the Yorkshire Dales.

Regional director Shaun Leavey said the proposed park could excite demand for more public access and even small numbers of people walking on farmland could be harmful.

He said: "National park designation creates a perception that this land becomes in a way nationalised and therefore people can go wherever they will.

"It is a difficulty which we have anticipated here but does exist in other national parks."

Farmers are concerned a national park would cause a surge in visitors and the stricter planning regime could hinder their ability to diversify and cope with changes to EU policies.

Mr Leavey said a "conservationist" national park could make it difficult for farmers to adapt and criticised conservation groups who wanted the Downs returned to a mythologised version of how they appeared in the Forties.

He said: "They imply there will be a mechanism for putting the clock back. They will be putting the clock back at a time when farming needs to be more dynamic."

The Countryside Agency said farming was vital to maintain the landscape and biodiversity, and viable agriculture in the Downs was in the national interest.

Barrister Robert Griffiths QC said the NFU had not objected to a national park in the New Forest, announced at the same time as the South Downs or suggested designation there would be incompatible with modern farming.

He said a ruling national park authority would have a legal duty to take farming into account.

The inquiry is due to run until September. The Environment Secretary's decision is not expected until 2005.