On 6 April the basic rate of income tax will fall from 22p in the pound to 20p. Charities can currently reclaim 28p of income tax for every pound donated by taxpayers through the Gift Aid scheme.
After 6 April, however, they will be able to reclaim only 25p in the pound.
"Charities need to communicate these changes to their donors in their fundraising materials," said Megan Pacey, director of policy and campaigns at the institute. "They will also have to calculate their Gift Aid claims at a different rate."
The institute is providing advice online, on CDRom and in its booklet Making Giving Go Further.