National Council for Voluntary Organisations fears cuts
Charities have warned cuts in local authority funding in England may threaten services they provide.
The National Council for Voluntary Organisations (NCVO) says some councils see them as a soft target for saving money and need to understand the impact these cuts have on vulnerable people.
The government has said cuts are unavoidable but other provisions are being made to help charities.
One in three pounds of charity money comes from government.
The voluntary sector magazine, Third Sector, estimates almost £13bn comes to charities from state sources.
Richard Kemp, deputy chairman of the Local Government Association, said: "We've known for the last 18 months that there would have to be big cutbacks in public spending to pay for the quantitative easing process.
"This is no surprise," he added.
The government has said cuts are needed across all areas but that it has freed up local authorities to work with voluntary groups in a new, more productive way.
Greg Clark, minister for decentralisation, said: "What we want to make sure we do is to make sure that there's space for voluntary organisations, for the 'big society', to take the initiative, to be able to say to councils, 'You were obliged to do this, this way by the government in the past, now let's sit down together and see if we can do things in a different way.'"
But the NCVO, which represents groups in England, says too many authorities are making arbitrary cuts across the board without recognising the value of the services provided.
Karl Wilding, of the NCVO, said: "All that we ask is that government cut with care, they cut with the knowledge of the impact of those cuts are going to be.
"Voluntary Organisations read the newspapers like you and I and they are expecting cuts in the region of 25-30% but our concern is that these voluntary organisations will be seen as a soft targets and they will experience bigger cuts than perhaps elsewhere."
The Chair of the Charity Commission, Dame Suzi Leather, said: "Charities have to be very realistic, have to think very hard about how to cut their costs, how to make the most of the money they have."
Andy Ricketts, of Third Sector, said the prospect of cuts was "a big concern for charities because, not only are they facing a reduction in funding, but they're also at the same time facing an increase in demand for their services as the recession bites, as unemployment rises".
A charity providing buses for other voluntary bodies in south London, Croydon Accessible Transport, fears it may cease operation after having its council funding withdrawn.
The charity's Robb Mackie said: "We are doing exactly what the government say they want us to do, we are the 'big society', we're doing it right here in Croydon - we're using a large level of volunteer support, people are helping each other, often for nothing, working long hours.
"Without the cash to actually run the minibuses, what we are doing may have to come to an end."
Martin Brookes, of think tank New Philanthropy Capital, said voluntary groups would need to prove their worth.
"There are some charities that are simply not doing good enough work or they can't prove how good their work is - they're more vulnerable than others," he said.
"The vital thing for charities to do is to invest in demonstrating their impact, proving that they deliver vital, valuable services and that they should be exempt from cuts."