A review into drawing-up national park boundaries could leave developers with a free hand to build on the fringes of the South Downs.
Land identified for industrial and commercial development in local plans could be excluded from the park, if the Government's chief countryside watchdog backs the review.
The review says sites earmarked for development at the time a park is designated should be excluded, unless there is a realistic chance the land could be restored.
Pro-park campaigners fear councils will exploit the move and push for land at the edge of the existing area of outstanding natural beauty (AONB) to be included in their local plans.
Paul Millmore, who is a member of the Sussex Downs Conservation Board, said: "It would be a way for local authorities to get bits excluded from the national park, bits they want to develop in the future."
In Brighton and Hove, the council's new local plan, which will guide its planning policy for the next ten years, is expected to be adopted in 2002.
A national park is unlikely to be designated before 2004, 12 months later than the watchdog first predicted.
Planners in Brighton and Hove are already considering earmarking sites for development inside the AONB, such as Falmer, Waterhall, Hangleton Bottom and Toads Hole Valley.
Elsewhere, the review could mean sites such as Upper Beeding cement works and the Halewick Lane waste transfer station, Sompting, falling outside the boundaries of the new park.
The Countryside Agency, which is due to discuss the review today, insists anywhere inside the existing AONB would still be protected even if it falls outside the park.
A spokesman for the watchdog said it was too early to predict whether councils would try and secure land for development on the fringes of the AONB.
But he added: "This is one way of interpreting what that says. It could well happen."
Draft boundaries for the new park will now be published in the middle of next year.
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