The number of web pages sending out malware has increased six-fold in under a year, according to security software company Sophos.

At the start of the year the firm was monitoring around 5000 malicious web sites a day. Now it is detecting approximately 29,700.

Internet users who visit any of these pages could have virus software downloaded onto their computer, even if they are only on the site for a short time.

Only one in five of the sites was designed specifically for malicious activity, with the remaining 80 per cent made up of legitimate sites that have fallen victim to hackers.

'With a whopping 80 per cent of all infected web pages found on legitimate sites, it begs the question as to why web hosts are not taking the necessary steps to properly secure their servers,' said Sophos senior technology consultant Graham Cluley.

'Simple measures such as keeping up to date with security patches will go a long way towards thwarting this problem - the fewer holes in server setups, the lower the risk of infection,' he said.

Over half of infected web sites (53 per cent) are hosted in China.