30 per cent increase in spam reported.

Security experts today warned of a "major new spam campaign" designed to manipulate the share price of a company that sells wireless products to young people.

Sophos reported that the scale of the spam campaign is such that it has led to a 30 per cent increase in the amount of spam seen by its global traps during the last 24 hours.

The spam messages are being sent to internet users worldwide with an attached PDF file that urges them to buy shares in a company called Prime Time Group. Investors may not be aware that the spammers have already purchased stock at a cheap price, and are now trying to artificially inflate its price by encouraging others to purchase more. The spammers plan to then sell off their stock at a profit, which may cause the price to plummet.

Sophos notes that since the rise in spam levels, Prime Time Group share prices have risen by 60 per cent.

The spike in spam was first witnessed at Sophos's spamtraps in Germany at 16:40 BST on Tuesday 7 August, but was quickly seen arriving at other monitoring stations around the world. The email messages are being sent from compromised home PCs, turned into zombies by hackers.

"The scale of this stock pump-and-dump spam campaign is like nothing we've seen before, and it looks as though it is working for the cybercriminals behind it. The share price in this company has rocketed as a result of bogus news being blasted to internet users worldwide," said Graham Cluley, senior technology consultant for Sophos.

According to Sophos, pump-and-dump stock campaigns currently account for approximately 25 per cent of the world's spam.