Imerys gave up rights to work in the Blackadown Valley

Crownhill Down covers 800 acres just outside the southern boundary of Dartmoor, near Plymouth.

The clay company Imerys has 50-year-old rights to quarry the area.

Now an alliance of conservation groups, including the Open Spaces Society, the Ramblers' Association and the Dartmoor Preservation Association is to campaign to persuade Imerys to give up its rights.

It is a most important wildlife reserve Dartmoor Preservation Association

Crownhill Down is outside Dartmoor National Park, but the land is considered by the conservationists to be of national park character.

The Dartmoor Preservation Association said: "Crownhill Down is important as a valuable area of open countryside - a true part of Dartmoor.

"The down has archaeology of international importance and is used a great deal by local residents as well as visitors from farther afield. It is a most important wildlife reserve."

Imerys said the clay reserves at Crownhill were vital to its business.

It had already made a significant gesture by giving up other rights to quarry in the Blackabrook Valley on Dartmoor after a similar campaign.

Imerys, formerly English China Clay, along with the company Watts Blake Bearne, had rights to quarry 400 acres of the valley and Shaugh Moor.

The rights, granted in 1951, were worth tens of millions of pounds because of the rarity and quality of the china clay deposits there.