Charlotte Townshend, owner of 15,000 acres of Dorset and a personal fortune estimated at more than pounds 60m, scuppered an agreement to give National Nature Reserve status to Dorset's Chesil Beach, the largest shingle bank in Europe, and The Fleet, the brackish lagoon it shelters.
The 17-mile long shingle ridge, which stretches from Portland to Bridport - and which Mrs Townshend owns - is one of the South Coast's most remarkable geological features and has high ecological value, with rare nesting birds, such as little terns, and important plant and marine life.
English Nature, the Government's conservation agency, has been negotiating with Mrs Townshend's Ilchester Estates for it to become the 200th National Nature Reserve in England, an event which was to be marked with an all- day celebration and lunch for 150 people in September.
But yesterday Mrs Townshend wrote to English Nature's chairman, Baroness Young of Old Scone, and said she would not enter into the agreement because of the Government's plans to ban hunting.
"You appreciate that our original motivation for the creation of the reserve was to ensure the long-term custody for the nation of an area of outstanding beauty and environmental importance," she said. "We regret to say, however, that the Government's astonishing and highly misguided proposal to ban hunting on English soil has destroyed our confidence in its understanding of countryside management issues."
English Nature said last night: "We are extremely disappointed."
Mrs Townshend, daughter of the 9th Viscount Galway, and formerly known as the Hon Charlotte Morrison, was regarded as the most eligible woman in Britain until her second marriage to James Townshend, a land agent, in 1995.
Besides her vast Dorset estates, she owns 3,000 acres of Nottinghamshire and parts of Kensington in central London. She lives in a country house at Melbury Sampford near Dorchester.
A keen supporter of field sports, she was at one time the companion of Captain Ian Farquhar, joint master of the Beaufort Hunt.
She is known in Dorset as a pleasant but strong personality, and although the letter to English Nature bears the names both of herself and her husband, she is the signatory.
She wrote: "Clearly our decision has not been taken lightly, particularly when the creation of reserve status might have conferred substantial benefits on The Fleet and Chesil Bank.
"We cannot stress enough, however, how profoundly we disagree with the Government on environmental, economic and social grounds, about the importance of hunting to rural landscapes and communities."