Acevo has written to David Cameron, leader of the Conservative Party, for assurance that a report criticising the voluntary sector by a right-wing think tank will not become party policy.
The Centre for Policy Studies' report into charities' operations, published last week, has also been criticised by the Charity Commission, the Institute of Fundraising and the NCVO.
Charity: the Spectre of Over-regulation and State Dependency, was labelled "flawed" and "simplistic" by Stephen Bubb, chief executive of Acevo.
But report co-author Philip Whittington, who is a researcher for the Conservative Party's social justice policy group, said the report was not intended to inform party policy or to criticise current practice among large charities, but to warn the sector about public concerns.
"The public is making comparisons between charities," he said. "People feel that large charities are not spending as efficiently as smaller charities."
The report recommends that charities providing public services should sever financial links with the state because the sector's vitality risks being undermined. It claims that public confidence in large charities is slipping because of high spending on fundraising and salaries.
Bubb said: "David Cameron has championed the sector's 'crucial role' in delivering services in partnership with the state. The report instead recommends charities are excluded from providing public services. This would demolish parts of the sector and damage government's ability to deliver its aims."
Bubb, who made recommendations to the party's social justice policy group last month, added that he was disappointed that Whittington was one of the report's authors, and accused him of proffering "simplistic and unworkable solutions to complex problems".
The report was also questioned by Sarah Atkinson, head of corporate affairs at the Charity Commission. "Developments in self-regulation have been ignored, with no acknowledgement of the Fundraising Standards Board, the ImpACT coalition or GuideStar," she said. "Such holes make me question the detail."
Laura Thomas, policy and standards manager at the Institute of Fundraising, said the report's criticism of fundraising costs was a red herring and that she hoped Cameron would look at other initiatives in the sector.
The Conservative Party said it was considering a range of contributions to its policy. Shadow charities minister Andrew Turner said no individual report would form party policy.