Computer users were warned of the potential hazards of Google today after the company admitted that criminals had hijacked links displayed in some lists of search results and attempted to install viruses on users' machines.

It was unclear how many people were affected by the attack, which targeted the sponsored links that appear on the right hand side of a page and involved the user being sent to a malicious website before being redirected to their desired destination.

Google would not say which search terms were involved, but according to reports, links associated with some 20 terms had to be dismantled, including the "Better Business Bureau", a US-based consumer organisation similar to Which?.

The offending links were shut down as soon as the problem was discovered on Tuesday, Google said, but the attack will cast doubt on the security of AdWords, the system the company uses to sell the advertisements which make up the bulk of its revenues.

AdWords allows companies to bid to have their link displayed prominently alongside the results thrown up by a particular search term.

In the attack, criminals bought links that appeared to be genuinely associated with search terms but which diverted users to a malicious site that attempted to install malware on their computers.

"The analogy would be if you were Reebok, you'd bid on the keyword 'Nike' and then place an ad that appeared to be a link to Nike but which sent the person searching somewhere else first," a Google spokesman said.

The company which discovered the threat, Exploit Prevention Labs, said that earlier this month it had run a Google search on the phrase "how to start a business".

The top sponsored link appeared to be from AllBusiness.com, a legitimate small business consultancy based in San Francisco, but the hyperlink actually led to a site that attempted to install a password-stealing program known as a "key-logger" on the user's PC.

"This discovery highlights problems facing all sponsored search vendors - how to determine the legitimacy of any individual advertiser, and how to determine whether a redirected link is being used legitimately," Exploit Prevention Labs said.

Computer security experts said that the attack, which was similar to one in 2005 that also targeted AdWords, appeared to be isolated, and affected only users of Windows XP who had not updated the anti-virus software on their machines.

In a statement, Google confirmed that its system had been infiltrated, saying: "This is an issue we’ve taken very seriously and will continue to monitor. We are evaluating our systems to ensure that the appropriate measures are in place to block future attempts."

Graham Cluley, a consultant at the security firm Sophos, said that the incident was indicative of a trend among virus-writers to use websites, rather than e-mail attachments, to spread malicious code.

"For many people Google is the internet, and just as in the past Microsoft's Internet Explorer and Outlook have been targeted, so too is Google becoming increasingly attractive for virus writers to try to exploit," he said.

Google recently reported that its first-quarter profits had risen by 69 per cent to $1 billion (£500 million).

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