While the government and computer firms have pledged to crack down on unwanted e-mails, the spammers are sending more than ever. In the past five months the amount of spam has almost trebled.

Daniel Druker, a vice-president at Postini, an e-mail security company, said: “The internet is under siege. Spammers are increasingly aggressive and spam has evolved from a tool for nuisance hackers to one for criminal enterprises.”

Postini detected more than 7 billion spam e-mails worldwide in November, up from 2.5 billion in June and making up 91% of all e-mails. Most big companies have spam filters but employees are still reporting increases as the spammers devise increasingly sophisticated methods to avoid detection.

Anna Newsome, who works for a charity association in Bristol, said: “I normally have 80 e-mails waiting for me each morning and 70 of them are selling Viagra or something similar. It takes me 10 minutes to go through my inbox.”

Ministers introduced regulations in 2003 to outlaw spam, but to date there have been no prosecutions. Microsoft successfully sued one Manchester man for £45,000 for sending pornographic spam, but he was not prosecuted in the criminal courts.

The latest wave of junk e-mails is blamed on new software that can hijack home computers to use them to send out spam. These networks are often not detected because each computer sends out only a small number of e-mails. They thus avoid being entered on the industry blacklist of computers sending spam.

The spam e-mails are also often modified to increase their chances of avoiding detection by spam filters. One simple ploy is to break up words such as Viagra, which trigger spam filter software, with punctuation marks.

Spam is also increasingly sent as picture attachments in e-mails. By inserting text into an image file, the spammers often get past detection systems.

According to AOL, which blocks 1.5 billion unwanted e-mail messages each day, the most common enticement to encourage users to open junk e-mails last year was “Donald Trump wants you, please respond”. Other top spam headings included “Body wrap: lose 6-20 inches in one hour” and “It’s Lisa: I must have sent you to the wrong site”.

Spam is used around the world to sell market products, promote pornography and con people into buying worthless shares.

“Phishing” attacks are also common, in which computer users are sent e-mails that purport to come from banks and request confidential passwords and banking details.

Some spammers are, however, facing justice. In September an American court upheld the conviction of Jeremy Jaynes, who was jailed for nine years for sending out hundreds of thousands of junk e-mails a day from his North Carolina home.

There is also some evidence that deluged computer users are fighting back. Nigel Roberts, a computer expert who lives in the Channel Islands, last year successfully sued a Scottish company that was sending him spam e-mails.

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