The amount of time that charities have to reclaim Gift Aid on donations will fall by two years from April 2010, HM Revenue & Customs has confirmed.
The rules currently allow charities to submit claims on donations made up to six years previously, but this is to be reduced to four from 1 April next year.
An HMRC spokesman said the change was part of a review intended to create a single regime for all direct taxes and was not targeted specifically at charities.
Barry Gower, managing director of Gain, a Gift Aid recovery consultancy, warned that most charities were not aware of the changes, which he said had not been widely publicised by HMRC.
"This is a wake-up call for charities," he said. "They need to check they have reclaimed all the Gift Aid they're entitled to."
Eric Grounds, director of fundraising at Sue Ryder Care, said many charities were not maximising the amount they could claim through Gift Aid, but the six-year reclamation period often meant more could be reclaimed.
He said that a review carried out when he joined Sue Ryder Care had revealed more than £700,000 in unclaimed Gift Aid, much of it dating back more than four years.
He said that if the reclamation period had been shorter when his review was carried out, it would have cost his charity a significant sum.
"The change in the rules will crystallise the loss of much unclaimed Gift Aid and make it harder for charities in the future," he said.