Europe's largest conservation charity, the RSPB, has slammed plans to build an eco-town near Weston-on-the-Green.
The charity has slated the 15,000-home Weston Otmoor proposal, which it claims would threaten nearby sites of special scientific interest and a nature reserve in the process.
Wendleybury Meads and Mansmoor Closes - a large group of meadows near Charlton-on-Otmoor - and Woodsides Meadow nature reserve, near Islip, are both rich in orchids, wild flowers and grasses, butterflies, moths and birds.
RSPB planning officer Colin Wilkinson said: "Our view on the Weston Otmoor eco-town is unequivocal.
"This site should be dropped now from the Government's shortlist.
"The Weston Otmoor proposal is a serious threat to one of Britain's natural jewels, a designated wildlife site bursting with unusual flora that is supposed to be protected from damage.
"Even if the developers could avoid destroying a whole section of this grassland by altering their development footprint, they cannot avoid inflicting human disturbance and changing the water table of this sensitive wildlife haven."
The RSPB's response to all 15 of the short-listed eco-town proposals highlights what it says are serious flaws in the process the Government is following, and raises urgent questions over the schemes' environmental credentials.
Mr Wilkinson said: "Most of the sites, including Weston Otmoor, raise significant transport issues."
He said the charity was disappointed with the overall sustainability of the proposals, particularly reducing people's need to travel, although it supported the concept of eco-towns in principle.
Mr Wilkinson added: "In the right place and with sufficient commitment from the Government, developers and local authorities, eco-towns could be remarkable examples of how we can create developments that help the environment."
He said the RSPB had put forward positive suggestions which would improve the quality of eco-towns and wanted the Government and developers to act on them.
Parkridge, the developer behind plans for Weston Otmoor, wants to build on 800 acres of farmland close to the A34/M40 junction - 28 per cent of the site lies within the Oxford Green Belt.