Natural England has agreed to designate all of Penwith Moors as a Site of Special Scientific Interest (SSSI), despite objections from some farmers and land managers.
The historic moorland landscape, located in west Cornwall, has been given the status in recognition of its national importance for wildlife.
Natural England said the SSSI would "protect an ancient landscape shaped over centuries through the careful stewardship of generations of land managers".
It means farmers and other land managers in the area must apply to the agency for consent to carry out certain activities on the land.
The area has a long history of livestock farming, with many of the 4,000 year old field systems still used for their original purpose.
However, in a response to the announcement, the Country Land and Business Association (CLA) said it was 'deeply disappointed' as Natural England "chose to ignore the concerns of farmers".
The designation had not taken into account the ability of farmers to make land management decisions in a timely fashion, the group said in a statement.
Natural England has not published a plan for how the site, which covers more than 3,000 hectares, should be managed.
The CLA said: "The lack of detail means that businesses do not know what they will be able to do once the designation commences.
“The decision fails to take into account any socio-economic factors. The impact of this SSSI designation will likely be disastrous for many."
Farmers and land managers will now have to apply to Natural England for consent to carry out every day activities, with fears the consent could be temporary, likely take a long time to be considered or may not be granted at all.
The CLA warned that many farmers within the boundary would have concerns as to the ongoing viability of their "already-stretched businesses".
"We call on Natural England to bring forward all management plans and agree sufficient funding arrangements with Defra well before designation is implemented," the group said.
“We also believe the designation of Penwith Moors as a SSSI lays bare some of the problems with the designation process more widely that need to be addressed."
