'Image spam' could bring the internet to a standstill

But this, experts say, is the spam that could bring the internet to a virtual standstill this year.

To bypass anti-spam software, the emails use an image instead of text.

In the past six months, this "image spam" has seen a massive increase and now represents 35 per cent of all junk email, according to security software firm F-Secure.

Mikko Hypponen, chief research officer, said: "Image spam is taking up 70 per cent of the bandwidth bulge on account of the large file sizes every single one represents."

The emails, generally containing stock tips, come from gangs and even bored teenagers in the United States and Russia trying to inflate prices in a swindle called "pump-and-dump".

They promise that a cheap, usually American, stock will take off. The perpetrator then dumps his stock as buyers leap in before it collapses.

Dmitri Allperovitch of computer security company CipherTrust said: "They're niche companies with no profit and no products, so when you see a spike from almost no trades to two or three million when the spam is sent out, you know there were a lot of people who fell for it."

Researchers believe hackers have targeted the recipients of computers at Christmas and are using these machines. Most spam is sent by "botnets" - machines hijacked by hackers to send out email on their behalf.

Some researchers estimate about 10 per cent of the world's 650 million online computers are botnet victims. They say this year has already shown just how large the networks after one of the biggest crashed, resulting in a 30 per cent fall in spam for the week it was disabled.

Diego d'Ambra of email monitors SoftScan said: "For years spam has just been steadily going up. A major botnet was almost certainly the cause of the fluctuation since botnets are now the single biggest factor in spam generation."

In the most notorious case so far, Jonathan Lebed, 15, from New Jersey, was prosecuted in 2000 for failing to disclose an interest in the shares he was promoting in chat rooms.

Lebed agreed to pay $285,000 (£150,900), which is thought to be significantly less than he made.

He claims to have gone legitimate with a stock-picking website.