A group representing 60 parish councils based around Oxford's Green Belt will fight plans to build 1,000 homes near the Kassam Stadium.
The Oxford Green Belt Network Group will demand that the scheme proposed for a site off Grenoble Road, near Oxford United's ground, is removed from the County Structure Plan, after members unanimously opposed the proposal at their annual meeting held in Summertown on Wednesday.
Group chairman Ian Scargill said: "The county council's plan to create a massive development close to Sandford and the Baldons is in contravention of Green Belt constraints." He warned that the development would lead to the "shredding of the whole Green Belt", making it impossible for it to turn down other developers. It would encourage landowners, including many Oxford colleges, to see enormous windfall gains from selling land at inflated prices, he added.
After the meeting, Mr Scargill said: "We have only three weeks to register objections to the plan. The Green Belt has benefited us all for more than 45 years. There may now be less than 40 days to save it."
The six-week consultation period on the planning blueprint for the county ends on November 7.
The Grenoble Road proposal is backed by Oxford City Council, which sees it as an answer to Oxfordshire's acute shortage of housing, but is being strongly fought by South Oxfordshire District Council, Oxford Preservation Trust and the Council for the Protection of Rural England.
Preservation trust spokesman Debbie Dance, who was at the meeting, said: "Once you have breached the Green Belt in one place, it will be a free-for-all."