Many communal charities are missing out on thousands of pounds a year because of a failure to claim Gift Aid on donations.
The lost income was revealed in a survey conducted by the Achisomoch Aid Company, which runs one of the largest charity voucher schemes within the Jewish community.
AAC trustee Michael Hackenbroch said: "It shows that not enough people are aware of the benefits of donating money through Gift Aid, both for them and for charities."
Gift Aid enables a charity to reclaim tax relief on a donation, effectively increasing the contribution by a quarter - a £10 donation becomes £12.50. All a donor often needs to do is tick a Gift Aid box on an appeal form.
The questionnaire was sent to 140 big and small charities across the community. More than a quarter responded.
Forty-three per cent of those reported that up to half their income came from donations without the Gift Aid claimed. The same proportion responded that they rarely received donations above £1,000, most commonly attracting amounts between £10 and £50.
Charity voucher accounts like those offered by Achisomoch - which had a turnover of £8 million-plus last year - already include tax relief in the voucher. They are "an easy way for people to gain tax relief on their charitable donations and to increase the amount of money they can donate to their preferred charity through Gift Aid," Mr Hackenbroch explained.