Sue Ryder Care is piloting a scheme that invites people who make donations of clothes in collection bags to complete Gift Aid forms.
The healthcare charity has produced a million clothing collection bags that contain leaflets explaining how the Gift Aid scheme works.
The leaflets also invite donors to take part by completing declaration forms, also enclosed in the bags, or by phoning the charity.
When the charity has sold £40 worth of their items, donors will be sent letters telling them that the charity will claim Gift Aid on this amount.
If a donor's situation changes or they change their mind, they have the opportunity to opt out of the scheme.
The charity hopes the scheme will increase the amount of Gift Aid it can claim on donated items and help it to build up a fuller picture of who is making donations.
Julie Beames, the charity's business development manager of retail, said the project was a first for charities running shops.