Opponents of a proposed new town that could include 6,500 homes close to the M40 have said it would be "disastrous" for Oxfordshire.
Developers want to build Harrington, which would be roughly the same size as nearby Thame, near junction 7.
But a Conservative MP and councillors have written to the prime minister and housing secretary to oppose the project.
It is not currently included in any official planning proposals.
South Oxfordshire District Council's Local Plan, which includes proposals for 28,500 new homes, will go to a public examination from 14 July.
But Bellway Homes, Pye Homes and Summix Limited are set to offer the site as an alternative if other proposed developments are not accepted by a government inspector.
John Howell, Conservative MP for Henley, along with South Oxfordshire district councillor Caroline Newton and Oxfordshire county councillor Stephen Harrod, wrote to Boris Johnson and Robert Jenrick to say the project was "not wanted".
"Harrington remains inherently flawed as a strategic site for housing development," Mrs Newton said.
"A town of the scale we fear is being considered would have an incalculable impact on this beautiful landscape, and, would be utterly out of scale with any other settlement in South Oxfordshire."
Mr Jenrick has previously told the district council that it must pass its Local Plan by December or risk him taking it over himself.
Conservatives worry that developments at Chalgrove Airfield and Culham might be dropped in favour of Harrington.
The developers have been approached to comment.
In April, they said Harrington "presents a unique opportunity" for South Oxfordshire.
"Our vision for Harrington focuses on delivering homes and services for the lifestyles we will be living in the next decade and beyond," they said.