Each donor's pledge will appear on a dedicated Double Your Donation website as well as on the website of their chosen charity. Supporters and members of the public will be invited to make donations to help match each pledge.
If public donations do not reach the target by a deadline set by the major donor, the major donor will instead match the total raised by the public.
"It is a kind of psychological incentive," said Dave Pitchford, founder of the project and managing editor of Intelligent Giving. "It's very satisfying for everyone because they have the drama of the countdown to match the pledge and they will in effect be doubling their donation to charity."
The site was due to launch this month, but was postponed after the first major donor pulled out at the last minute.
The idea comes from the US, where a survey by economic researchers Dean Kaplan and John List showed that donors were 20 per cent more likely to donate in this way and were also likely to give 20 per cent more.