Charities could be benefiting from fraudulent donations made by credit card thieves trying to test the validity of stolen cards.
According to high-street banks, fraudsters often make low-value transactions, such as a one-off donation to a charity, to see if a stolen card has already been blocked or reported missing.
An industry source, who asked not to be named, said banks were "less likely to recover fraudulent funds from charities than from major commercial organisations that deal with vast sums of money". As a result, such funding would often pass through unnoticed, the source said.
A spokeswoman for the Nationwide Building Society said: "There are certainly instances in which fraudsters will make small payments, including charitable donations, to check that a card is working before going on to make much larger purchases."
She said that, without carrying out extensive research, it was impossible to determine exactly how many of these payments were made to charities.
A spokesman from Apacs, the trade body for organisations that process payments, said not every bank had identified a link between low-value transactions and credit card fraud.
However, if a bank spotted a connection between a particular series of low-value transactions and fraudulent activity, it would change the criteria its computer systems used to detect fraud.
The system might then treat a one-off charitable donation as an anomaly in the cardholder's spending habits, he said. The card would be blocked until the bank confirmed the payment was legitimate.
Douglas Campbell, a charity management and disability consultant, said his card was blocked for a week after he made a donation through the website Justgiving. When he contacted his bank, he was told it was just being cautious.
"But for fundraisers like me, this is a worry," he said. "If a donor finds using their card to make a donation results in it being refused for other transactions, they will find this inconvenient and will stop making such donations."