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

It was unclear how many people had been affected by the attack, which had 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 about 20 terms had to be dismantled, including the Better Business Bureau, a United States-based conusmer 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 that the company uses to sell the advertisements that 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 in fact diverted the user to a malicious site that tried to install “mal-ware” on the machine.

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

The top sponsored link appeared to be from All-Business.com, a legitimate small business consultancy based in San Fransisco, but the hyperlink actually led to a site that tried attempted to install spyware on the user’s PC.

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similar to one in 2005 that also targeted AdWords, appeared to be isolated, and affected only users of Windows XP with out-of-date antivirus software.

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