The Government could see charities and voluntary organisations as a "soft target" for cuts, the NCVO has warned.
The State and the Voluntary Sector, a report published today by the umbrella body, says the sector receives £12bn of its £33bn income from statutory sources.
About 25,000 voluntary organisations receive more than three-quarters of their income from the state, making them particularly vulnerable to cuts, it says.
"The fear is that the sector could be seen as a soft target for cuts," an NCVO statement accompanying the report said.
Stuart Etherington, chief executive of the NCVO, said: "Public sector funders must avoid a knee-jerk reaction. Ill thought-out cuts would ravage communities across the UK.
"When developing spending plans, government – nationally and locally – must understand the vital contribution projects run by voluntary and community organisations make."
Voluntary organisations that provide social services receive more income from the state than any other sub-sector, followed by employment and training organisations, according to the NCVO.
Seb Elsworth, director of strategy at chief executives body Acevo, said public service delivery was more about partnership than patronage and continued funding depended primarily on voluntary organisations being able to demonstrate the value of their work.
A Cabinet Office spokesman said: "This is a Government committed to delivering public services through the third sector and that has a record that puts the third sector at the very heart of public service delivery."
The report costs £25 (£17.50 for NCVO members) and is available from www.ncvo-vol.org.uk/publications.