Kerbside collection bag companies have been warned to be upfront and clear with households about how much of their donation actually reaches the named charity.

Those behind the charity collections must make it clear that they are commercial enterprises, and that only a proportion of the profits from the sale of donated goods will make it to the charity, under new guidance from the Committee of Advertising Practice (CAP).

CAP said the new guidance followed a review of potentially misleading advertising by private door-to-door collection companies, and comes after the Advertising Standards Authority found a company had not been clear enough about the commercial nature of its business last year.

Problems have included companies giving households the misleading impression they are donating directly to a charity when they are not, failing to provide their company name or commercial status on the bags, and giving undue prominence to the charity's name and registration number.

Businesses who are unable or unwilling to follow the guidance have been warned that they face being referred to Trading Standards.

CAP said it was aware of concerns about charity bags left on doorsteps by "fly-by-night or bogus operators", but said these were best tackled by law enforcement bodies, adding that anyone concerned about the legitimacy of a collection service should check with their local council or donate directly to an official charity.

Companies have been given until June 2 to bring their collection bags in line with the new guidance.

CAP's director of the committees, Shahriar Coupal, said: "No-one should feel duped into thinking they are donating directly to a charity if that's not the case.

"Appeals to consumers' generosity can benefit a range of good causes but it's only fair that these companies are truthful and transparent about the commercial nature of the service they provide.

"Our guidance sets out clearly how commercial collection companies can stick to the rules which will help build consumer trust and, ultimately, benefit charities."