Spammers are sending out viruses that take over people's computers and turn them into junk mail machines, say experts.
Spam accounts for about half of all e-mail sent globally
It is a worrying new development in the ongoing war against spam and could see a huge rise in the amount of unsolicited messages in inboxes worldwide.
In a worst case scenario, it could overwhelm the entire internet system warn experts.
UK spam filtering firm MessageLabs has found what it believes is the first example of a virus that has been sent by a spammer.
Back door to spam
Spammers are hijacking the internet system to effectively destroy the internet with pornography and other illegal messages
Matt Sergeant, MessageLabs
Around half a million copies of the AVF virus were sent out to computer users in a two-day period last week and MessageLabs is convinced a spammer is to blame.
Unlike other mass-mailing viruses AVF does not mail itself out to everyone in the infected computer's address book. Instead its open up a backdoor in the machine through which spammers can send out junk mail.
As well as using the infected PCs for sending out e-mails about porn sites, miracle cures for baldness and get-rich-quick schemes, the system ensures almost total anonymity for the spammer because it is virtually impossible to trace the route back.
Users are advised to keep their anti-virus software up to date to guard against the malicious trojan.
"It is a terribly worrying phenomenon," said Matt Sergeant, MessageLab's senior anti-spam technologist.
"It pushes spammers from being a nuisance to a whole new level of being completely illegal. Sending out viruses to people shows they have no regard for law whatsoever," he told BBC News Online.
Victims of a computer hijack are unlikely to realise that they are unwittingly playing the role of middleman to the spammers.
Locally drafted laws are useless. We need internationally accepted anti-spam legislation
Julian Bogajski, Sybari Commercial Director
"Users we have told have been astonished that their computers are being used in this way. The only thing they might notice is that their internet connection slows down a bit," said Mr Sergeant.
Spam now accounts for around half of e-mail sent globally and is becoming more than just a headache for users.
Around 65% of spam is sent on hijacked computers, using insecure software already installed on peoples' machines, said Mr Sergeant.
The new trend in actively sending viruses that open the backdoor of the computer could have a devastating effect, he warned.
"Spammers are hijacking the internet system to effectively destroy the net with pornography and other illegal messages," he said.
"It will definitely cause an increase in the amount of spam we see," he added.
Spam is becoming a growing problem for both web users and internet providers.
The content filtering firm Sybari has called for co-ordinated global action against the menace of spam.
It wants governments worldwide to join together and come up with anti-spam legislation that can be enforced in every country around the world.
"Locally drafted laws are useless. We need internationally accepted anti-spam legislation, " said Commercial Director at Sybari Julian Bogajski.
"The only other alternative is to accept the current state of affairs where spam is costing businesses serious amounts of money as e-mail ceases to become a productive means of communication," he added.
In Europe alone it is calculated that spam costs businesses in excess of £2.5 billion each year.