STOP the Weston Otmoor eco-town' is the plea from local conservationists urging people to write to their MPs to protest at the plan.
Berkshire, Buckinghamshire and Oxfordshire Wildlife Trust has issued postcards saying "Stop" and is asking people to fill them in and send them to the Minister of State for housing, communities and local government.
Chief executive Philippa Lyons said the eco-town posed "a devastating threat to an area of Oxfordshire that is of national importance to wildlife".
She said the proposed development of 15,000 houses, 10 schools, shops, businesses and a station would mean the loss of the trust's Woodside Meadow nature reserve, and part of the Wendlebury Meads and Mansmoor Close site of special scientific interest, with one of the largest complexes of wildflower meadows in the country.
Ms Lyons added: "The trust is not opposed to eco-towns in principle or the need for more housing if it is in the right place."