Charities will find it simpler to claim tax relief on phone and web donations from next month, Gordon Brown has said.
The scale of UK donations after the tsunami highlighted the issue
They will no longer need written consent from donors to claim Gift Aid - worth 28p per pound - from the government, the chancellor announced.
Rules were relaxed following the Asian tsunami, with the UK's Disasters Emergency Committee allowed to include Gift Aid details in general letters.
It had been feared UK groups could lose £9m in Gift Aid from tsunami donations.
Mr Brown said in a speech in Edinburgh on Friday: "A few months ago the tsunami showed the world not only the power of nature to destroy but also the power of humanity to build anew.
"It is in recognition of the extent of immediate giving by phone and internet that we are making it easier for donations to... charities, even when made by telephone, to attract tax relief and thus extra money for the cause."
Institute of Fundraising Chairman Joe Saxton was quoted by the Times as saying: "Many charities are effectively having to waste donors' money by writing to people who have made a telephone donation.
"Now they will be able to spend that money on the charitable causes for which their donations were meant."
The new rules will come into force on 1 November.