One in 10 web pages scrutinised by search giant Google contained malicious code that could infect a user's PC.

Researchers from the firm surveyed billions of sites, subjecting 4.5 million pages to "in-depth analysis".

About 450,000 were capable of launching so-called "drive-by downloads", sites that install malicious code, such as spyware, without a user's knowledge.

A further 700,000 pages were thought to contain code that could compromise a user's computer, the team report.

To address the problem, the researchers say the company has "started an effort to identify all web pages on the internet that could be malicious".

Phantom sites

Drive-by downloads are an increasingly common way to infect a computer or steal sensitive information.

They usually consist of malicious programs that automatically install when a potential victim visits a booby-trapped website.

"To entice users to install malware, adversaries employ social engineering," wrote Google researcher Niels Provos and his colleagues in a paper titled The Ghost In The Browser.

"The user is presented with links that promise access to 'interesting' pages with explicit pornographic content, copyrighted software or media. A common example are sites that display thumbnails to adult videos."

The vast majority exploit vulnerabilities in Microsoft's Internet Explorer browser to install themselves.

Some downloads, such as those that alter bookmarks, install unwanted toolbars or change the start page of a browser, are an annoyance. But increasingly, criminals are using drive-bys to install keyloggers that steal login and password information.

Other pieces of malicious code hijack a computer turning it into a "bot", a remotely controlled PC.

Drive-by downloads represent a shift away from traditional methods of infecting a computer, such as spam and email attachments.

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