Cancer Research UK and mental health charity Mind have reported that donations of goods brought straight to the shop are on a downward spiral. Sales have not fallen, leading to diminishing stock levels.
The problem is forcing shops to consider what they can do to drive up donations and increase stock levels.
Frank Hawkins, head of resources at Minds Matter, a subsidiary company that manages Mind's shops, said: "We have seen some slow-down in donations and we are considering our options all the time."
Regular customers were continuing to use the shops, but were not bringing in their old clothes, said Hawkins. It therefore needed to find different ways of attracting donations.
"We can appeal locally through the shops or employ house-to-house agency collectors, but that's not really a great proposition because it puts pressure on our margins," he said.
David Moir, head of policy and resources at the Association of Charity Shops, said charities must be proactive and educate the public.
"We must promote the benefits of donating, or this trend might gain a momentum of its own and charity shops will have nothing to sell," he said.
Simon Ledsham, trading director at CRUK, said charities should create new ways for the public to give rather than waste money on collectors.
The fall in donations was happening because of changing lifestyles rather than as an early effect of the economic slowdown, said Ledsham.
"It is getting more difficult for people to donate to shops because they are just too busy," he said.
"We should make it easier for people to donate on their own terms, perhaps in their workplaces or by extending shop opening hours, rather than getting more vans and people on the road.
"That comes at huge cost and you become very reliant on it."
Ledsham said charities were also missing out on a huge amount of clothing that gets thrown out and sent to landfill sites.
"We need to increase people's awareness to get them giving to charity shops instead."
- There are more than 7,000 charity shops in the UK, which generate more than £550m a year
- £110m goes towards parent charities and £440m is spent on running costs
- 93 per cent of goods sold in charity shops are donated by the public[QQ]- Shops must sell mainly donated goods to maintain charitable status
Source: Association of Charity Shops.