The government must not water down the community right to buy, nine sector umbrella bodies have told Eric Pickles, the communities secretary.
Sector organisations including Acevo, Navca and the NCVO said in an open letter they were concerned about proposals to reduce the amount of time for community organisations to exercise the right to buy from six months to three.
The proposals the letter refers to are an expected amendment from the Country Land and Business Association to the Localism Bill, currently being considered by the House of Lords.
The right to buy in the bill would give community groups the right to register land and property as having community value. When that asset came up for sale, they would have a period of time to put in a bid to buy it.
The bill sets the moratorium at six months, but Chloe Stables, parliamentary and media manager at the NCVO, said charities were aware of a "concerted lobbying effort" by landlords and landowners to reduce it to three months.
The letter says the signatories would have "serious concerns should the proposals be watered down and the moratorium reduced".
It says: "All the evidence from our members suggests that even six months is a very tight timeframe for community groups to confirm their interest, assess viability, raise the finance and deal with any legal or other complications.
"Three months, in nearly all cases, is likely to prove impossible."
Last week, the communities umbrella body Locality, another signatory to the letter, published a briefing document on the bill that said reducing the moratorium would "make nonsense of the entire provisions".
The Communities and Local Government department issued a statement that said: "The Localism Bill includes new rights for local communities, including the community right to buy and the community right to challenge.
"The detailed approach will be established through regulation and guidance. To assist in developing the detail we have consulted on both rights.
"The consultation process has now closed and the department will be responding in due course."
The full list of signatories is:
- Sir Stephen Bubb, chief executive, Acevo
- Steve Wyler, chief executive, Locality
- Kevin Curley, chief executive, Navca
- Sir Stuart Etherington, chief executive, NCVO
- Debbie Sorkin, director of engagement, National Skills Academy for Social Care
- Peter Holbrook, chief executive, Social Enterprise Coalition
- Toby Blume, chief executive, Urban Forum
- Justin Davis Smith, chief executive, Volunteering England
- Tim Waldron, chair, YMCA England