Charity shop donations are being stolen from doorsteps at a "record rate" as the price of textiles soars and thieves look to make money during the recession.
Many charities have seen takings leap as shoppers look for a cheaper alternative to high street stores but donations have also plummeted, with theft contributing as a factor.
The Children's Society estimates one in 10 filled donation bags are being swiped by crooks from doorsteps before charity workers can collect
them.
The PDSA said the number of bags stolen from doorsteps had "more than doubled" in the last six months compared to the same time last year and the British Heart Foundation (BHF) said theft was at a "record level".
Ken Blair, chief executive of BHF Shops, said: "We believe this is the tip of the iceberg.
"For every call reported we believe there are another 20 that go unreported. If that were the case then we estimate that we would be losing around 1,000 tons of donated goods per annum valued at around £2.5m.
"There has been an increased number of reports of bogus van collectors operating in the UK. These collectors are working for commercial gain, with none of the proceeds going to charity.
"Household appeals are a vital source of income for our BHF Shops. Taking bags that have been left out for our charity is akin to robbing people with heart conditions of a better quality of life."
Mr Blair said the charity was receiving between five and 10 calls a week regarding donation bag theft compared to one or two a month at the same time last year.
PDSA director of business services Andrew Holl said every bag stolen was taking "vital funds" away from pets in need of vets.
He said: "The charity has seen a drop of 6,000 bags collected since January - a portion of this is undoubtedly down to theft.
"Bogus collectors are a real problem, as they are depriving the charity of thousands of pounds.
"It's also deeply distressing for our supporters when they realise that the charity they have elected to support loses out. The number of shops reporting bag theft is at a record high."
Charities are urging the public to contact them directly if they wanted to make donations securely or take them directly to their shops. The public can also ask collectors for identification.