Local authorities from across the West Midlands fear they will not be able to meet Government house building targets without encroaching into the green belt.
The warning contradicts assurances given by regional assembly chairman David Smith that as many as 576,000 new homes could be accommodated by 2026 with-out building on the most sensitive sites.
Coun Smith (Con Lichfield) described claims by the Campaign to Protect Rural England that the green belt was under threat as "mischief making", and said the possibility of developing on protected land was "virtually nil".
But last night the CPRE hit back, releasing details of house building forecasts submitted to the regional assembly by councils. CPRE spokesman Gerald Kells, who is a member of the regional assembly, said that Coventry, Warwickshire, Solihull, Herefordshire, Shropshire, Staffordshire and Worcestershire had all concluded that it would not be possible to meet the 576,000 target without taking green belt or green field land.
Doubts were also expressed about whether the assembly's strategy of concentrating house building on urban brownfield sites would work.
Mr Kells predicted that developers would not be willing to pay to clear and develop such sites and would "cherry pick" green field locations instead.
He added: "We very much support what the regional assembly is doing.
"However, all the evidence from the local authorities suggests that achieving Government targets would mean l arge scale green field development. Some local authorities are saying there w ould be green belt development.
"There are plans for massive green belt incursion to the south of Coventry."
Mr Kells said Herefordshire Council and Shropshire County Council believed they could not meet the target without moving away from sustainable development and threatening the environment.
S taffordshire County Council said meeting the target would "raise serious questions about the integrity of the green belt".
Stoke-on-Trent Council warned that land in Staffordshire not protected by the green belt was of such poor quality that no one would want to build on it.
Worcestershire County Council is planning large-scale expansion of Kidderminster, Redditch and Bromsgrove, including the possibility of encroachment into the green belt. The city of Worcester would be expanded toward the environmentally sensitive Malvern Hills.
Mr Kells added: "The CPRE is not scaremongering. We have analysed what the local authorities are saying.
"We do not want a row with the regional assembly. We are bringing to the attention of the Government the implications of what it is doing."
Coun Smith remains convinced that housing targets in the West Midlands can be delivered without building on the green belt.
He said: "If local authorities want to make submissions as part of the regional spatial strategy review then these will be considered line by line.
"On behalf of the regional assembly, I can categorically state that we have no intention of breaking into the green belt."