A dangerous strain of 'virus spam' is tricking computer users into allowing serious infections into home and business computer systems, IT experts have warned.

According to industry body The Corporate IT Forum (Tif) virus spam, or 'v-spam', dodges antivirus and firewall systems by tempting users to click on a website link contained in an email which then sends them a virus.

Tif said that incidents of v-spam are increasing at the same rate as spam, which has doubled over the past year.

While any virus can be delivered in this way, Tif believes that the key logger virus, which silently sits in computers recording key strokes and bank and credit card details, could be one of the most dangerous.

"Clicking on a link in a spam email is the equivalent of handing a burglar the keys to your house," said David Roberts, chief executive at Tif.

"People must understand that there could be a very nasty shock lurking behind each and every spam email.

"If you open up a link in a spam email, it may be days or weeks before you know you have a virus. You are not going to get flashing lights telling you what you have done."

Popular versions of v-spam are thought to be emails offering the chance to unsubscribe to bogus e-newsletters by clicking on a link; messages with links to 'unbeatable offers'; and emails tempting users to download festive greetings cards.

Billions of spam emails are delivered each day, most sent with the aim of attracting surfers to buy products or services from the internet.

While many users think that spam is intrusive and offensive, it is often considered more of an irritation than a threat - until now.

"The strain of v-spam means that computer users and the government, when setting legislation, must take the issue of spam a lot more seriously as the viruses they can let in can cause damage to both home computers and business systems," Tif warned.

Roberts added: "The message is clear: be alert, always think before you click and never be fooled by great deals sent by email."