A public inquiry into the creation of the South Downs National Park re-opens on Tuesday.
Campaigners have warned that an area of iconic English landscape almost as big as the Isle of Wight could be at risk if a revised boundary for the park is accepted.
Sir Martin Doughty, chairman of Natural England, said: "This is the final chapter in securing a long-awaited South Downs National Park, but if the story is going to have a happy ending, the Western Weald must be included in the designation.
"Natural England has always maintained that the smaller boundary is the wrong decision and it is high time we awarded the South Downs the elite National Park status it deserves.
"We need to get this right."
Following the public inquiry, the planning inspector will submit proposals to the Environment Secretary who will make the final decision on the park's boundary.
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