Accountancy firm Grant Thornton said a tax anomaly will allow higher-rate taxpayers to claim tax back if they fill in a Gift Aid form before entering museums, zoos and stately homes. But the rebate will not be available to basic-rate taxpayers.
The discrepancy has emerged following Chancellor Gordon Brown's compromise formula on Gift Aid on admissions.
Pulling back from closing the loophole entirely, Brown said charities would still be able to claim Gift Aid on entrance fees provided the visitor made an extra donation of at least 10 per cent.
Heritage and conservation charities are bracing themselves for loss of revenue, but Michael Rogerson, head of Grant Thornton's charities and not-for-profit group, said the benefit to higher-rate taxpayers could soften the blow.
They will be able to claim back the difference between the basic (22 per cent) and higher (40 per cent) rate of tax on admission charges if they pay the extra amount to qualify for Gift Aid. An admission charge of £5.50 would only cost higher-rate taxpayers £4.23.
"The initial reaction to this scheme was that it would be difficult to entice visitors to part with an extra 10 per cent," said Rogerson. "But if charities can show visitors in the higher tax bracket that they can save money by giving an extra 10 per cent, both sides stand to benefit."
But Rogerson said the anomaly was unfair on ordinary families.
Helen Wilkinson, policy officer at the Museums Association, said museums were unlikely to promote the tax advantage to higher earners. "It's too complicated," she said. "It is also against the spirit of our organisations that rich people should get a better deal than poor people."
She added that charities affected by the Gift Aid changes were involved in informal discussions with the Treasury concerning the benefits that could be offered to encourage visitors to pay the extra 10 per cent.
£15m a year - The value of Gift Aid on admissions to heritage and conservation charities
10 per cent - The minimum visitors will have to pay on top of admission charges for Gift Aid to continue
£1.27 - The amount higher-rate taxpayers can claim back in tax on a Gift Aided entry fee of £5.50
2006 - The year the changes announced in this year's pre-budget report will start.