Charities are losing £1m a year because householders are mistakenly donating unwanted goods to bogus collectors, the Office of Fair Trading and the Association of Charity Shops have warned.
The trading watchdog and the umbrella body are concerned that some donors are being conned by unscrupulous companies that employ door-to-door collectors who give the impression that they are from a charity.
Some companies also distribute flyers that promise to send unwanted goods to needy people in poor countries. However, the goods never make it to the intended cause, but are instead sold to generate profits for the company.
The OFT is appealing to the public to be extra vigilant, and charities have been advised to make their supporters aware.
"We introduced a kitemark scheme four years ago," said Liz Gaffer, a spokeswoman for the Association for Charity Shops. "Members of the association can use the kitemark on their collecting sacks to distinguish genuine charities from bogus collectors."