The amount of stock received by charity shops from doorstep collections has dropped by more than half since 2008, according to figures from the Charity Retail Association.
The CRA has blamed the fall on the rise in the number of bogus collections, which is estimated to cost charities £50m a year.
Research published today by the association found that 18 per cent of stock donated to charity shops came from doorstep collections in 2011/12, compared with 52 per cent five years ago.
The figure for last year was down from 24 per cent in 2010/11, 27 per cent in 2009/10 and 40 per cent in 2008/09.
In a statement, the British Heart Foundation said it had reported 241 incidents of theft of donation bags from doorsteps in 2011/12, but added that it believed it was just "the tip of the iceberg".
Louise Harbour, who manages 14 British Heart Foundation shops in London and the south east, added: "At times, we’ve seen whole areas being targeted by bogus collectors who follow our vans on their collection routes and steal donations residents leave on their doorstep for us."
Some bogus collectors are known to work in organised criminal gangs, which sell the clothing abroad for profit. CRA research has found that the same gangs are also involved in other serious crime including trafficking, violent crime and fraud.
CRA is launching a campaign ahead of the police and crime commissioner elections on 15 November, which is calling on candidates to pledge to tackle bogus collections.
Warren Alexander, chief executive of CRA, said: "House to house donations to charity shops are at an all-time low. There is more than one reason for this, including more competition from businesses which sell clothing for profit. But the increase in bogus collections is one of the key factors.
"As only one in seven of people making donations actually check on the bag to see it's from a genuine charity, it's no surprise that these gangs are taking advantage of the generosity of households."