Beautiful countryside could be "lost forever" if parts of Hampshire and Sussex are left out of a South Downs National Park, campaigners have said.
Bill Bryson is president of the Campaign to Protect Rural England
The claim has been voiced by author Bill Bryson, president of the Campaign to Protect Rural England (CPRE), who made a fact-finding visit to the area.
Mr Bryson said he was "shocked" 371 sq km of land in East Hampshire and the Sussex Downs could be omitted.
A public consultation on the national park runs until 24 September.
The area was first designated as a national park in 2002, followed by a two-year public inquiry.
The process was put on hold in 2005, and then restarted in July but with the boundaries of the proposed park redrawn.
The CPRE said a planning inspector had recommended reducing the overall size by 23%, with the East Hampshire and Sussex Downs areas of outstanding natural beauty (AONBs) left out.
Mr Bryson said: "I was shocked to see the planning inspector's argument to exclude a large tract of land known as the western Weald.
"If this beautiful landscape is left out, it will run the risk of being picked off by developers and lost forever.
"It has also been recommended that the historic market towns of Petersfield in Hampshire, and Midhurst, Petworth and part of Arundel in Sussex, be excluded.
"Where is the sense in excluding these settlements which have such strong historic and cultural links with the South Downs?"
The proposed national park runs from Eastbourne to Winchester
The CPRE wants Environment Secretary Hilary Benn to return to the original wider boundary.
Spokeswoman Emma Marrington said the western Weald would not necessarily benefit from any greater protection under national park status, but explained that the current scenario would leave it exposed.
She said: "It's very likely it would face de-designation as an AONB (Area of Outstanding Natural Beauty), because you couldn't justify it remaining as a smaller one if it's not beautiful enough to be included in a national park.
"Developers could potentially have a field day and the landscape would really suffer."
Ms Marrington added that the main benefit of national park status was more funding towards promoting recreational enjoyment of the areas.
The smaller park is also being opposed by the South Downs Campaign, the Council for National Parks and Hampshire County Council among others.