Recession-hit consumers are opting to sell their unwanted goods to second hand clothing companies that offer around 5 per bag of clothes - rather than give free donations to stores run by charities.
Companies such as Cash4Clothes - which has seven outlets across the Lothians, Fife and Angus and is looking to expand further - sell the clothes on at discount prices through stores in poorer countries in Africa and Eastern Europe.
But not-for-profit organisations have warned that they do not provide value for money and have urged people to donate to charity shops instead.
Retail guru Mary Portas, who pioneered the upscale Living and Giving brand of charity shops for Save the Children and was last week named the government's retail tsar, said: "The idea of charity shops is that you buy something and you give something back.
"Consumers must start giving back to society. This is what charity and charity shops are about, it's not just about commercialism but community."
She added: "Shopping at charity shops is not only good for you, it's good for our world and the part we all play in it."
Auction website eBay has previously been blamed for taking business from charity shops, while second hand designer goods stores sprang up during the recession to rent and sell high-end wear to cash-strapped fashion lovers.
However, the latest wave of stores specifically target the mid-range goods favoured by charity shops.
"It is like the new version of eBay for us, but worse," said Isabelle Adams, projects and policy officer at the Charity Retail Association, which represents more than 300 charities including Age Scotland and the Bethany Trust north of the Border.
"People in the past have found they can sell some of their clothes online, but they only usually do it if they have a couple of special items they want to get rid of. But these kinds of stores are a way for them to make some money out of whole bags of clothes."
Adams added that the rising value of textiles had made it easier for firms to make money out of old clothes. Recent figures released by Zero Waste Scotland showed that unsorted bags of clothes, some of which could be sold on and the rest recycled, can now fetch up to 800 per tonne for collectors - three times the price of two years ago.
"It is amazing what these companies can get for the clothing they receive," she said. "But people are not really getting much from them for their donations."
But the clothing recycling companies claim charity shops are oversubscribed with donations and insist that they are not diverting goods from their stores.
Brendan Scott, recycling manager at Cash4Clothes, said: "We work with some charity shops, we take the goods they don't want and get rid of them for them."