Charities Aid Foundation claims its not-for-profit banking arm CafCash has generated over £17 million of extra revenue for charities since April last year.
The estimate is based on the difference between the rate of interest for CafCash accounts and the average of the best rates offered by other banks.
CAF says that the extra interest generated between April 2002 and April 2003 will be between £17 million and £18 million. It forecasts a further windfall of £20 million for the coming financial year.
CAF's head of banking Peter Mitchell said: "Over the next four years, we estimate CafCash will generate over £100 million in interest. That figure is based on the difference in rates of interest and doesn't take into account savings on commission charges."
CafCash, launched 17 years ago, now manages £800 million of UK charity funds.