Change will affect about 12% of Google search queries in US in move designed to tackle so-called 'content farms'

Google is making a "pretty big" change that will demote "low-quality" or "shallow" websites from online search engine results, in a move designed to tackle so-called "content farms".

The change, which will affect around 12% of Google search queries in the US, follows pressure from the media industry and many of its users.

Although Google did not specify which sites would be affected, the search engine has come under fire for allowing content farm sites like Demand Media - which produces thousands of articles a day based on popular search terms - to "pollute" its results.

"This update is designed to reduce rankings for low-quality sites - sites which are low-value add for users, copy content from other websites or sites that are just not very useful," said Amit Singhal, a Google fellow, and Matt Cutts, head of the company's spam-fighting team, wrote in a blog post late on Thursday.

"At the same time, it will provide better rankings for high-quality sites - sites with original content and information such as research, in-depth reports, thoughtful analysis and so on."

The move is Google's biggest yet in responding to growing criticism over the relevancy of the world's most popular search engine. It vowed to address the concerns in January.

Last week Google launched an extension to its Chrome web browser allowing users to set up a "personal blacklist" of sites that would no longer appear in their search results. Google said 83% of the "top dozen or so" sites which most often featured on the blacklist were demoted with its algorithm change.

Responding to Google's announcement, Demand Media's executive vice president, Larry Fitzgibbon, said: "As might be expected, a content library as diverse as ours saw some content go up and some go down in Google search results.

"This is consistent with what Google discussed on their blog post. It's impossible to speculate how these or any changes made by Google impact any online business in the long term - but at this point in time, we haven't seen a material net impact on our content and media business."

The move will also be seen as part of Google's wider attempt to woo news organisations and other "high-quality" content producers. Some publishers' content had slipped down Google search results as content farms rose in prominence.

Last week Google unveiled plans for its One Pass online charging service for newspapers and magazines, just a day after Apple unveiled a rival internet payment offering for publishers.