For a donation of 拢5 a month, individual members of the Key2Giving scheme will receive an engraved key fob, which is branded on one side with a charity's logo and has a Freepost address on the other side.
If anyone finds the keys, they can be dropped into a letterbox and returned to their owners through the postal service.
Deputy managing director of Fundraising Initiatives, Anne Bolitho, said: "The problem with establishing a committed programme for small charities is that it can involve substantial investment upfront, for example with direct mail or DRTV.
"Key2Giving will be self-financing for charities, and it is also responding to the public's changing attitudes towards giving to charity, whereby they increasingly expect more choice and something tangible in return for their gift, which is especially true of younger donors these days."
A pilot trial with mental health charity Guideposts Trust, with donors recruited through face-to-face and door-to-door fundraising, will continue, with Childlife, Pattaya Orphanage Trust, The Connection at St Martins, and the Rainbow Trust Children's Charity also signed up for the scheme.
Dual membership at 拢8 for two key fobs and household membership at 拢12 for up to four fobs is also available.
Guideposts Trusts' head of funding and development, David Beck, said: "As well as recruiting more donors at a higher value, we will be aiming to upgrade donors and will also be looking for an increase in retention and loyalty to Guideposts.
"Every time supporters use their keys they are seeing the Guideposts Trust logo and that's increasing our brand loyalty. When we mail them in future we are hoping for a higher return because of the higher recognition factor Key2Giving will deliver."
Key2Giving is similar to a key recovery service run by the Canadian armed services charity War Amps since 1946.
More than one million sets of lost keys have been returned to Canadians through War Amps' 'Key Tag' service since the start of the scheme.
Bolitho said: "The War Amps system proves that this works in practice and isn't just a theory."
Charities will also be able to attract donors through a microsite on the website.
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