The National Free Wills Network will offer will-writing services by a network of up to 800 Law Society-approved solicitors.
Charities will pay fees for membership and to the solicitors who will write simple wills for supporters in the hope that they will leave legacies to the charities concerned.
Stephen Maund, campaign director of the National Free Wills Network, said: "The network opens up the possibility for charities to be more specific in the targeting of their legacy approach and to explore the hitherto virgin territory of the high net worth individual donor who has yet to make a pledge."
But experts have warned that charities must be aware of the potential pitfalls.
Richard Radcliffe, chairman of legacy giving agency Smee & Ford, said that free wills services might "reek of bribery and blackmail" to some supporters.
"People think that where there's a free will, there's a catch," he said. "People wonder 'why is a charity willing to write my will for me?' And some think it cheapens the passion to give through a legacy."