Banks and credit card firms have been exposed as Britain’s worst junk mail offenders.

They are responsible for more than a third of the 3.4 billion mailshots that are sent out each year, of which more than 750 million items are thrown away without being opened or read.

According to a survey by Neilsen Market Research, the worst offender is the finance company MBNA, which sends out 99 million mailshots a year at a cost of £44.5 million. Third on the list is Capital One, which dreams up 20 new pieces of junk mail each month. But despite public antipathy to junk mail, which costs £1.87 billion a year, the survey suggests that four in every five mailshots are opened and that people are more likely to read junk mail from a bank than any other type.

Marketing magazine, which commissioned the research, said: “The financial services industry has long been accused of carpet bombing households with hundreds of millions of unwanted credit card and loan offers. Yet the study found consumers are more likely to open those mailings than letters sent by mail order companies and charities.”

The study asked 10,000 households to record the junk mail they received and what they did with it.

They said that they immediately threw away mail if it was addressed to the wrong person or contained spelling mistakes, binning about 22 per cent of all junk mail unopened.

However, they opened about 85 per cent of junk mail from banks, compared with 70 per cent from mail order companies and charities.

Even though junk mail elicits responses from 1 per cent to 3 per cent of recipients, companies believe the cost of a mailshot is well spent as mail itself raises brand awareness.

However, Marketing said that the study suggests that mail order companies and charities may need to review their marketing strategies. It believes the success of finance companies is linked to the use of more sophisticated data to identify potential customers.

Arjan Duijk, the marketing director of Capital One, said: “We send to less than 15 per cent of the mailable population in the UK. We make use of our data through hard work and smart work.”

The study follows the dismissal of Roger Annies, a postman from Barry, South Wales, who helped his customers block junk mail. He was offered a job in a sorting office after a nationwide outcry at his treatment.

Householders can prevent junk mail by registering online to optout@royalmail.co.uk, but the Post Office says the service may also stop important local notices from arriving.

MBNA Europe said yesterday that it had not released any figures for the study. “We are a large organisation without a branch network and we rely on a number of channels to promote our products to potential customers,” a spokesman said.

“Direct mail is just one of them. Our direct marketing practice is not simply volume driven. It is highly targeted, highly sophisticated and highly effective.”