The figures in an Institute of Fundraising campaign to ensure that charities and donors can reclaim all the tax on donations by higher-rate taxpayers are being challenged by HM Revenue & Customs.
The institute's campaign says the Government keeps 40 per cent of the tax when these donors make Gift-Aided donations.
But when Third Sector asked HMRC about the figures, a spokesman said that the institute's calculation was based on "an incorrect method for how Gift Aid works". He said 100 per cent of the tax paid could be reclaimed, either by the donor or the charity. Higher-rate taxpayers were entitled to a rebate, which meant they could get back a quarter of the money they gave, he said.
For example, if a donor gave £80 to a charity, that charity could claim £20 in Gift Aid and the donor would get a £20 rebate. The final cost to the donor would be £60, the value of the donation to the charity would be £100 and no tax would be withheld, he said.
But the institute does not consider a higher-rate taxpayer's rebate as a reduction in the original donation. It calculates that, on an £80 donation, a total of £20 should be given to charity and £33 returned to the donor, instead of the £20 in the above example. The institute considers that the Government keeps £13 for itself.
Lee Grant, tax-effective giving project manager at the Institute of Fundraising, said: "We're still working on our proposals with HMRC to simplify the Gift Aid system."