The Office of Fair Trading has issued a warning to households in Britian about unscrupulous traders masquerading as charity collectors.
The trading watchdog said that genuine charities are losing £1 million a year because people mistakenly give unwanted belongings to con artists.
Bogus collectors are issuing flyers promising to distribute unwanted belongings to poor countries, but the OFT claims the flyers give the misleading impression that the items are being collected for charitable purposes.
The companies involved are believed to make a profit by selling off donations abroad.
In some instances a registration number is quoted in the flyers, but instead of being a registered charity number, it is often a limited company number.
People who have doubts about a collector are being urged to contact the Charity Commission.
Christine Wade, director of the consumer regulation enforcement division at the OFT, said: "Bogus collections deprive genuine charities of significant sources of revenue.
"We want people to be alert to the practices of unscrupulous individuals and companies that seek to exploit their generosity and goodwill."
Antony Robbins, head of communications at the Charity Commission, said: "We're asking people to be generous but be smart and don't let bogus collectors put you off giving to charity.
"Check for a registered charity number and feel free to check the number with us."