From Wednesday 6 April, charities including The Christian Institute will lose 3p for every pound donated by Gift Aid because of changes to Treasury rules.
The amount of Gift Aid that charities can claim back on donations will drop from 28 per cent to 25 per cent.
The change was put in place by the previous Labour Government, but it comes into force next week.
In a year’s time, the move will have cost the charity sector an estimated £100 million in lost revenue.
Donations to charities, including The Christian Institute, are therefore worth more if the donation is made before Wednesday 6 April.
Director of The Christian Institute, Colin Hart, said: “Every penny of every donation is precious to us, and here at the Institute we strive to make sure that every penny counts.
“If you are a UK taxpayer and are thinking of making a donation, please do so before 6 April and make sure the donation is Gift Aided if applicable.
“The drop in Gift Aid will hit our future income, so if people are able to raise their level of giving, that would be a big help.
“We are so grateful to the thousands of ordinary Christians and churches who support our work financially, and through prayer. We could not do our work without them.”