Spam which advertises share issues has rocketed by 700 per cent last month, according to security firm Clearswift.

The company explained that the share tips are part of so-called 'pump and dump' scams that were popular in the late 1990s during the internet boom, and have continued ever since.

'Pump and dump' involves boosting interest in the shares, causing the stock price to rise and allowing the spammer to make a profit.

"The spammers clearly enjoyed a certain degree of success with these scams and it seems they're coming back again for another pop," said Alyn Hockey, director of research at Clearswift.

"Even if the 'insider tip' is rubbish, the surge in interest means an increase in value; it becomes a self-fulfilling prophecy. You would not invest your hard-earned cash on the advice of a man in the street, and the same should apply to emails from people you've never heard of."

One reason for the popularity of financial spam is that junk mail filters often work by identifying offending key words, while email containing business terminology is unlikely to be filtered as thoroughly.

Over a quarter of all spam detected by Clearswift in April was related to financial offers. The increase came at the expense of pornography, which dropped to just five per cent for the month.