A community group formed to prevent inappropriate development on Putney Common has begun legal action against the organisation who are supposed to protect the greenspace.
Solicitors representing The Friends of Putney Common (FoPC) have sent a 'letter before action' to the Wimbledon and Putney Commons Conservators (WPCC), the formal start of High Court action.
Daggers were drawn after the FoPC claimed the WPCC do not have the power to grant the 'easement', or permission, for Wandsworth Council to build private access roads on the common so a proposed 420-pupil school and 24 luxury flats can be constructed on the site of the Putney Hospital.
FoPC, who are formed from Putney residents, have asked the conservators to agree to terminate what it considers an "unlawful agreement" and unless they do so the group intend to seek a ruling in the High Court.
A spokesman for FoPC said "We are very disappointed that through their actions WPCC have forced residents to take this legal route to protect the common, a duty of conservation laid out in the 1871 Act that they should be carrying out.
"We have sent a series of legal opinions demonstrating that the proposed easement is not lawful and expected the WPCC to react accordingly. They have not done so."
A spokesman for the WPCC declined to comment on proceedings. Earlier this year, it was announced that planning consent for the new school would be quashed forcing the council to reapply. Last month part of Putney Hospital collapsed forcing the local authority to bring in demolition experts.