The Association of Charity Shops has told charities that use commercial agents to carry out doorstep clothing collections that their activities are a significant threat to fundraising.
The association sent a strongly worded letter to charities to warn them against using agents to collect and sell bags of clothing directly to rag merchants. A percentage of the sale price goes to the charities.
The letter, seen by Third Sector, says that clothing generates more profit in charity shops and diverting the income to other organisations "could have a devastating effect on charity shops' ability to raise funds".
Children's charity the Make a Wish Foundation was one charity to get the letter. Karen England, fundraising director of the foundation, said the points it raised were misguided and inappropriate.
"Putting pressure on charities to force them out of the marketplace seems wrong," she said. "Shops are just one fundraising model and we use another - just because charities adopt different methods doesn't mean one is right and one is wrong." England said the association was protecting larger charities that could afford to run shops, but clothing was a vital source of unrestricted income for those that couldn't.
A spokesman for collection agency Clothes Aid said the association should consider matching its own demands for transparency by putting statements in charity shop windows showing the income raised from clothing donations and the percentage that goes straight to recycling agents.
David Moir, head of policy and public affairs at the association, said such measures were unnecessary. "Charity shops are happy to explain exactly what they do," he said. "The public knows that many of their clothes cannot be sold. I am happy with the level of transparency."
Moir said the association was not telling charities how to fundraise, but it was important for transparency reasons that the issue was discussed openly.
Lindsay Boswell, chief executive of the Institute of Fundraising, said the institute supported the association on the issue.