The 21 charities based in Palingswick House, west London, have accused Hammersmith & Fulham Council of "going against the big society rhetoric" by proposing to sell the building and offering it as the site of a new school.
The Conservative-run authority has written to the charities and community groups that rent the building from the council, warning they are likely be asked to leave.
The council has issued a public statement saying the site is one of two locations that "may become available" for the West London Free School, which is being set up by the author and journalist Toby Young.
A statement issued yesterday on behalf of the 21 charities said: "If the council decides to sell this building, along with several others throughout the borough, many of the voluntary groups could face closure. The council is going against the big society rhetoric by kicking out community groups."
Bruce Marquart, a project manager at the Palingswick House-based charity Upper Room, which works with homeless people and ex-offenders, told Third Sector: "The council has given us very little information about what is happening.
"We've been told we may be relocated to a 'community hub'. But the so-called hubs are really the ground-floor entrances to tower blocks on housing estates, and none of them are ready for use yet."
In a statement, Stephen Greenhalgh, the leader of Hammersmith & Fulham Council, said: "Having looked at the detailed consultation results and my officers' assessments of the impact of the proposals, I am minded to support the sale of the site, subject to the discussion at cabinet on 7 February.
"If we can do that and provide a home for a new school, offering new choices for local parents, I would be delighted. It is my view that local people would rather the site was returned to its former use as a school serving the local community than for residential development."