“Pump and dump” spam now accounts for almost a fifth of unsolicited messages, up from less than 1 per cent in 2004. But the growth in the share-tout spam is predicted to have peaked.

The scam has been a lucrative one for organised criminals. The average return for a spammer who invested in touted stock was 6 per cent over two days, though in some cases the amount was much higher, according to a recently published report by an Oxford University academic. If the spammer invested $10,000 in a number of heavily touted stocks, they could make $133,770 over several two-day periods, it found. The average loss for the victim was 8 per cent.

Jonathan Zittrain, the chairman of internet governance and regulation at Oxford University, analysed more than 25,000 pieces of stock-tout spam sent to a single inbox over 18 months.

“Unlike most spam, which calls for a response through the network, for instance the recipient clicking on a link to buy Viagra, stock tout scams call for an external response. That makes it much harder to identify those responsible,” he said.

Graham Cluley, an analyst at Sophos, a computer security company, said that in some campaigns the share price rose by as much as 600 per cent. “With stock spam, there’s no outlay. You just pick a company and send the e-mail. If it doesn’t work, you can always just sell the shares at the price you bought them.”

The stocks touted are “penny stocks”, costing less than 50 cents per share, and are listed on smaller exchanges in the US. While some are legitimate companies, others appear to have been deliberately created to lure investors with promises of imminent breakthroughs in technology or mining.

The US Securities and Exchange Commission has charged a couple in Connecticut after they made more than $1 million from buying 288 million shares in a penny stock.

The Financial Services Authority said that a survey of 100 victims of such scams — typically older, more experienced investors — found that each lost on average £20,000.

The spammers may have become a victim of their own success. It appears that investors are now so wary of companies promoted in spam that the share price of touted companies immediately drops, leaving the spammers with a loss.

Ways to avoid spam mail

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