Stephen Bubb, head of chief executives body Acevo, has written to the Prime Minister, Gordon Brown, urging him to revise the Government's policy of making the NHS the preferred provider of health services.
Bubb said the policy, set out last year by the health secretary, Andy Burnham, had "serious implications" for the Government's vision of supporting the development of the voluntary sector.
"We believe this new policy of treating the NHS as a preferred provider is a direct breach of the Government's manifesto pledge to treat the third sector on equal terms," Bubb wrote. "We urge you strongly to reverse this policy."
The letter was written after the Co-operation and Competition Panel for NHS-funded services decided not to take further Acevo's complaint about a primary care trust excluding charities from bidding for community services contracts.
Acevo had hoped the panel would provide more clarity on the issue.
The panel's decision not to proceed prompted the shadow health secretary, Andrew Lansley, to ask the Office of Fair Trading to investigate Burnham's policy.
"It is clear that NHS organisations are continuing to apply the policy and extending it to a wide range of procurements," Lansley wrote to the OFT. "In doing so, they are acting in a way that is to the detriment of patients across the country."
An OFT spokeswoman said: "We can confirm we have received the letter and will respond in due course."