Nearly 90% of charity bosses fear the government's cap on tax relief for charitable donations will hit their wealthy backers, as the backlash against George Osborne's budget continues.
According to a survey of 120 charity chief executives by the Charity Aid Foundation (CAF), 88% said the move, announced in last month's budget, will have a "negative impact."
CAF's Chief Executive John Low said the survey showed "widespread alarm and despair" in the charity sector.
Nearly nine out of 10 - 89% - said they were concerned about the move's impact. More than half believe major donations will fall by as much as 20%.
It comes after more than 2,000 charities and individuals signed up to support the Give it Back George campaign against the so-called charity tax.
In the wake of David Cameron promising to listen "sympathetically" to charities' concerns, Low said: "We need the government to embrace and celebrate our modern philanthropists for the good of society as a whole. It was good to see the prime minister this morning confirm his commitment to encourage charitable giving."
The survey comes in the wake of several charities expressing their concern over the cap, which the government has claimed is necessary to help crack down on tax avoidance.
The NCVO's Deputy Chief Executive Ben Kernighan said Number 10's comments represented a "remarkable slur" and were "extraordinarily unhelpful."
The prime minister's official spokesperson defended George Osborne's claim that the cap would prevent "abuse" of the system, saying on Tuesday that "in certain instances" people "may be giving to charities" who don't "do a great deal of charitable work" to avoid tax.
"I think that on the one hand you have got a government which has been talking about the importance of charities and trying to promote philanthropy and now we get this remarkable new line of attack.
"You've got a Charity Commission whose purpose it is to regulate charities. If the government doesn't think that charities are being regulated well, they should do something about it, rather than trying to cut off an enormous amount of funds. They are presenting no evidence of this being a significant problem and they are also using an enormous sledge hammer to crack what appears to be a bit of a small nut," he told The Huffington Post UK.
Tory MPs also tweeted their concern, with backbencher Zac Goldsmith and parliamentary private secretary to Northern Ireland secretary Owen Paterson Conor Burns saying: