The chief executives of umbrella bodies Acevo and the NCVO, and the heads of some of the country's largest charities, have signed an open letter calling for reform on Gift Aid rules for higher-rate taxpayers.
Higher-rate taxpayers are able to claim for themselves the difference between the basic and higher rates of tax on donations, provided they fill out a form. They can then pass this on to charities.
But the institute says almost half of those eligible do not fill in the form because it is too complicated.
The letter calls on the Government to allow charities to claim the Gift Aid, rather than the higher-rate taxpayer.
"It ought to be easier for fundraising organisations to benefit from the additional tax relief available to higher-rate taxpayers," it says. "And therefore government should introduce changes which would allow charities to claim the full amount of tax on the donation that is made by higher-rate donors.
"We believe this would act as a real incentive to higher-rate taxpayers, who would see that their money could have a much higher impact."
The letter comes after a petition on the 10 Downing Street website calling for a simplification of the rules on higher-rate taxpayers and Gift Aid.