Combating virus and worm attacks is costing internet service providers around the world millions of dollars, with the figure rising every year as broadband is more widely adopted.

According to network equipment provider Sandvine, ISPs are shouldering a massive burden in order to minimise these attacks on residential subscribers.

Yesterday home users were blamed by antivirus company MessageLabs for being the most vulnerable to attacks because of poor security on home PCs.

"While every sector has been impacted in the past year, it is no surprise to see that those worst hit by both viruses and spam have high levels of interaction with home users," said Mark Sunner, chief technology officer at MessageLabs, in a statement.

Sandvine added that on any given day up to 12 per cent of all internet traffic across service provider networks is malicious, with denial of service attacks now an everyday event for ISPs.

The main problem for ISPs is the amount of data traffic that such attacks generate. This includes not only large-scale attacks, but persistent, low-level attack traffic caused by remnants of previous worms that tenaciously cling to residential PCs.

Sandvine said the cost to ISPs was in the thousands to millions of dollars in unplanned network and customer support costs, draining ISP profit margins that are already pared to the bone.

The firm has put the overall global figure for this year at $370m, most of which ($245m) will be borne by ISPs in the US.

"If ISPs didn't have to spend so much time and money on viruses and worms, although the entry-level price of broadband would remain roughly the same, their technical departments would be better placed to roll out new services and consumers would get better value for broadband," said Ian Fogg, analyst at Jupiter Research.

And Duncan Ingram, managing director of BT Openworld, said: "Preventative measures and cures are available but we must take some individual responsibility. An ISP such as BT Openworld makes free antivirus software available to its consumer customers."