Millions of householders are being duped when making donations to door-to-door charity bag collectors, a leading charity claims.

Just under a third (30%) of items donated to charity via letterbox bags actually stand a chance of ending up in high street charity shops – with most of it sold abroad for private profit, according to research by the British Heart Foundation (BHF).

Many charities, often those without shops, do deals with commercial firms who collect door to door for them with bags emblazoned with the charity logo. But the companies keep the donated goods and sell them for profit, mostly to overseas markets. They then make a royalty payment to the charity, but as little as 5% of the cash made goes back to good causes, according to the BHF.

In some cases charities are getting £50 to £100 a tonne of goods collected when, in fact, the goods can sell abroad for anything up to £1,800 a tonne. This is now a lucrative industry, with householders inundated with charity bags as firms chase millions of pounds of profits from the trade.

The poll by the BHF showed 70% of the charity bags an average householder receives are from commercial companies.

But 65% of those polled were not aware these commercial companies exist and mistakenly thought all of the money raised goes to good causes.

When householders were told of the arrangement, the BHF said 85% of them felt "shocked, cheated and disheartened".

The steep increase in charity bag collections has led to an estimated loss of donations direct to BHF shops worth £4.6m in the past two years, the charity said.

Mike Lucas, retail director for the BHF, said: "It is vital commercial companies act responsibly and be transparent on their charity bags – particularly around how much profit the named charity will actually make from a collection. Householders have the right to know what happens to their donations and currently this information is not clear."

He said that although this was a legal way to raise money companies working for commercial gain were a huge problem for charities with high street shops. "BHF shops do not work with these companies and because of this 100% of the profits made from charity bag donations stay with the charity – helping us continue our life-saving work."

The BHF released the findings in support of their Big Donation campaign for September, appealing for good-quality clothing, shoes, accessories, CDs, DVDs, books, toys and bric-a-brac to stock their 670 shops nationwide.

It said people should take donations directly into BHF shops and that householders should check charity bags and leaflets for information about how much of the proceeds from items go to the charity.