The Disasters Emergency Committee and Revenue and Customs have reached a one-off compromise that will enable Gift Aid to be claimed on telephone donations to the tsunami appeal.
The DEC had feared that it would not be able to claim the tax relief because of the administrative requirement to write to all donors who had asked verbally for Gift Aid to be applied.
Under government rules, charities must inform such donors in writing that they intend to claim Gift Aid.
But the scale of the response to the tsunami meant the DEC was faced with the prospect of writing to upwards of 800,000 people.
However, following the intervention of Chancellor Gordon Brown, special dispensation has been given to the DEC. It will now be able to notify donors of the Gift Aid claim as part of a "thank you" letter that it planned to send out anyway.
However, the Charities Tax Reform Group said it would be lobbying the next government to change the rules so that charities do not have to confirm Gift Aid on phone donations in writing.
"The process is absolutely unnecessary," said CTRG administrator Helen Donoghue. "It doesn't add value."