Hampshire County Council has objected to the new boundaries proposed for the South Downs National Park.
The South Downs stretches from Winchester to Eastbourne
A revised map of the park published in July by the government leaves out Liss and Petersfield.
The council said the changes were "fundamental" and would remove 300 sq km of land from the northern boundary of the proposed national park.
A public consultation is now under way, which has been extended to 24 September.
The changes, which also saw Midhurst and Petworth in West Sussex excluded, means the South Downs National Park will only include chalk downlands.
The council said this fails to recognise the natural association the chalk landscape has with the adjacent farmland.
It said a variety of landscape character was an important factor in the "overall amenity" of the park.
Councillor Ken Thornber, leader of Hampshire County Council, said excluding the area could lead to it losing its status as an Area of Outstanding Natural Beauty (AONB) and see it miss out on government funding.
"The land has been managed in an integrated way with the land that will remain in the national park and it makes no sense to separate it out just because it is not a chalk landscape," he said.
Campaigners have fought for 60 years to make the area stretching from Winchester in Hampshire to Eastbourne in East Sussex a national park.