The draft new Compact "significantly weakens" commitments to the voluntary sector, according to the Commission for the Compact.
In its submission to the consultation on the draft new document, the commission highlights the absence of measures to protect small charities, as well as the removal of commitments to support voluntary sector infrastructure.
It also expresses concern about a commitment for voluntary organisations to campaign "responsibly" and based on "robust evidence".
"What constitutes 'responsible' campaigning is highly subjective – it may be that what an impartial observer regards as responsible could be held by government to be irresponsible, especially if the government is on the receiving end of the campaigning activity," the commission's submission says.
The Compact, which outlines how the voluntary and public sectors should behave towards each other, was established in 1998 and updated last year.
The coalition government and Compact Voice, which represents the voluntary sector on Compact issues, are in the process of renewing the agreement.
They hope the new version will be published in November in time to protect charities from any effects of next week's comprehensive spending review.
The draft contains 37 key principles, compared with 95 in the existing document.
"The new Compact simply commits government to 'support' the sector, with no indication of what that 'support' might be," the commission says. "'Support' is open to wide interpretation, so the nature and extent of the support put in place in pursuit of this commitment will differ widely. We believe that this significantly weakens the commitments contained in the current Compact."
The commission, which last month was included on a leaked Cabinet Office list of quangos to be abolished, led consultation on last year's refreshed Compact. However, Compact Voice is in charge of the latest consultation.
A Compact Voice spokesman said: "Some of the commission's points we've also heard from the voluntary sector during our current consultation, and other points are at odds with what the sector has been saying.
"And while they have produced good analysis regarding some aspects of the renewed Compact, others parts of their analysis we don't agree with."