About 30 people are expected to be made redundant under a wide-ranging restructure at the National Council for Voluntary Organisations.
The umbrella body has begun a consultation with its 117 staff over proposals that put 66 jobs at risk but will also create 37 new positions.
The organisation said it expected its annual income to fall from £10.4m to about £8m.
The bulk of the cuts and job losses are due to the closure of Capacitybuilders, the non-departmental public body that provides funding for infrastructure services and had contracts with the NCVO to operate support services in three work areas.
The NCVO is also due to lose at least £500,000 because of the reshaping of the Strategic Partners Programme, run by the Office for Civil Society. This will be cut from £12.2m in the current year to £7.5m from April next year, with no organisation receiving more than £500,000: the NCVO gets nearly £1.1m.
In a statement, Sir Stuart Etherington, the chief executive of the NCVO, said he was "very sorry that, because of reductions in public funding, some posts will cease to exist".
"Over the last 18 months we have been consulting with our members, staff, assembly and partners about what our priorities for the future should be in the context of reduced public funding," he said. "We have taken a fundamental look at what we do, how we do it and how best to represent and support the sector into the future."
The umbrella body will also take out a £1.8m loan to build an extra floor on its north London building so it can rent out the space to charities. This move, if given the go-ahead by planning authorities, is expected to be completed in 2012.
Ben Kernighan, deputy chief executive at the NCVO, told Third Sector the move made financial sense.
"It will provide a useful service to voluntary organisations and will ensure that we practise what we preach," he said. "We believe that charities should make the best use of their assets and this will help generate income that will strengthen our independence."
He said the NCVO had nearly paid off its mortgage on the property and the loan was prudent at a time of low interest rates.
As part of the restructure, the NCVO said it planned to launch a new parliamentary advice service for members, expand the direct support it provides through consultancy staff and increase the support it offers on public services and the Compact.
The staff consultation will run until 7 January. The new structure will be in place by 1 April.