Online fundraising organisation Justgiving.com is to introduce a £20 a year charge to charities.
Until now, the privately-owned web site has offered online donor services free of charge.
According to Justgiving director Anne-Marie Huby, the average online donation is £40 so only one a month would make the investment worthwhile.
She said: "There has been tremendous growth in demand for our services in recent months. Subscription fees will enable us to sustain our current level of service and deliver further innovations."
The web site processes around £3,000 of donations each day.