Greenfield sites across Brighton and Hove are at risk of being lost forever, according to Friends of the Earth.
The campaign group urged Brighton and Hove City Council to “pull back from making a serious mistake” when it debates its “urban fringe” policy tomorrow (Friday 11 July).
It added: “This is before the proposals have been scrutinised and the sites allocated for development.”
The council’s Policy and Resources Committee is to discuss the proposed changes at Hove Town Hall tomorrow afternoon.
Councillors are being asked to approve the changes as part of a package of amendments to the draft City Plan, a blueprint for land use in the area up to 2030.
They were asked by a government planning inspector to look again at whether greenfield sites around the edge of Brighton and Hove should be kept as open spaces or converted into housing.
The council is under pressure to allocate more land for housing to tackle the shortage which has left 15,500 people on the local waiting list.
Friends of the Earth said that it understood the pressure that the council was under to get the City Plan approved.
It “also recognises that there will be some sites in the urban fringe suitable for development”.
But the environmental group added: “In the haste to proceed, the council could make a terrible mistake.”
Chris Todd, from Brighton and Hove Friends of the Earth, said: “The council’s proposed new policy would allow development to take place on any site listed in the Urban Fringe Assessment as soon as Part 1 of the City Plan is adopted.”
This would, he said, in effect set in stone the right to develop all of these sites before the case for doing so had been scrutinised.
Mr Todd added: “The council’s promise that there would be a consultation on allocating the sites at a later date is worthless.
“The council needs to step back from the brink and amend what it is proposing.
“Otherwise it will be making a grave mistake and sites wrongly assessed in the Urban Fringe Assessment could be lost forever.”
A demonstration is expected to take place outside Hove Town Hall at 1.30pm with the meeting, which is open to the public, due to start at 2pm.