Alan Strickland, senior policy and information manager at Volunteering England, said it was pleased to see "some big, ambitious plans for volunteering" from the government, which had talked about every adult as an active member of a community group.
But he warned: "We're concerned that it's too ambitious and the government needs to have a bit of a rethink."
Strickland said volunteering organisations lacked the resources to support volunteers and had been hit by a huge fall in funding from local authorities and primary care trusts, as well as a drop in support funding from central government.
"We found that councils were particularly reliant on local agreements and early intervention grants to fund volunteering," he said. "It is cuts to these grants that have hit our sector worst."
He said he expected several volunteering funds to be announced in the Giving White Paper, due out around the end of May, but that these would go "no way at all" to replacing lost funding.
Strickland also said he felt the government's plans for the 'right to challenge', which would allow groups of local volunteers to bid to take over the running of public services, would be difficult to achieve.
"We saw with the previous administration that asset transfers from the local council often ran into problems when volunteers realised the difficulties of running these buildings without council funds," he said. "We think this could be replicated here."
He said he did not expect the right to challenge to be as widely taken up as the government predicted, because many local groups were wary of the public accountability involved in running services such as parks and libraries.