The number of computer viruses in circulation has reached one million for the first time, according to a report by a leading security firm.

Symantec said that just over 711,000 new viruses were identified in the past year - an increase of 468 per cent on the number identified in the previous year.

The US was still by far the greatest contributor to the pool of malicious software infecting the world's computers, accounting for 24 per cent of such activity. Madrid, meanwhile, was the city with the highest number of so-called 'zombie' computers - machines that have been co-opted into performing particular tasks by criminals based elsewhere.

In total, Symantec observed an average of 62,000 zombie computers operating every day in the second half of the year - an increase of 17 per cent on the previous period.

The government sector was responsible for most 'identity exposure' resulting from the theft or loss of personal information from databases, accounting for 60 per cent of the total in the past six months. The education sector was the second worst perpetrator, accounting for 24 per cent, and healthcare the third, with 16 per cent.

An increasing amount of malicious code is infecting computers via vulnerabilities in web browsers such as Internet Explorer and Safari, Symantec suggested in its bi-annual Global Internet Security Threat Report. In the second half of 2007, 88 vulnerabilities were reported in Mozilla browsers, such as Firefox, 22 in Safari, Apple's browser, and 18 in Internet Explorer, the report concluded.

In the previous period Internet Explorer was subject to 39 vulnerabilities and Safara 25, while only 34 were reported in Mozilla browsers.

The total number of unique phishing messages - e-mails designed to lure bank customers to a fake website where they are encourage to hand over personal details - also grew by 5 per cent, to 208,000, Symantec said.

That equated to 1,134 new messages for each day in the second half of the year, with about 66 per cent of all phishing websites designed to look like companies in the financial sector. Social networking sites are also falling victim to phishers, with two unnamed networking sites accounting for 91 per cent of all phishing attacks which used websites based in the US.

China is now the second-ranked country - behind the US - for hosting phishing websites, accounting for 4 per cent of the total - compared with 1 per cent in the previous period, when it was ranked 18th.

One possible reason for the increase was that there had been a rise in phishing scams and other fraudulent schemes attempting to exploit the popularity of the Olympic Games, which are being held in Beijing in August, the report said.

China had also emerged as the new base of the Russian Business Network (RBN), a shadowy organisation which specialises in the distribution of malicious code, but which virtually disappeared in November last year after a campaign by police and other investigators, the report suggested.

The RBN has been credited with devising approximately half of the phishing scams conducted worldwide last year.

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