Wikipedia has started to instruct search engines to ignore links on its pages which point to external websites. 

The user-created encyclopaedia has started to include 'nofollow' tags in all external links.

This prevents the links from being spidered by search engines, or used to determine a website's popularity by mechanisms such as Google's PageRank. 

Wikipedia took the action in response to a search engine optimisation contest in which webmasters were challenged to gain the highest ranking with major search engines for the query 'Global warming awareness 2007'.

One of the contestants created a spam entry on Wikipedia which included a link to his own webpage.

The 'nofollow' tag was first introduced by companies providing blogging services in an effort to curb the flow of spam links in comments on blogs. 

Brion Vibber, a web developer who works on Wikipedia, announced the changes in an email to a Wikipedia developer email list on Saturday. 

He suggested that the block was a temporary measure and called on developers to create a better way to filter out spam links using heuristics or manual flagging.

Wikipedia's enormous popularity, and the fact that any user can add and edit content, has made it an attractive target for spammers.

While obvious misstatements are typically caught quickly, less obvious ones can remain unnoticed for long periods of time.

Some encyclopaedia authors applauded the move as an effective measure to curb spammers, but it drew criticism from others.

Business author Nicholas Carr said on his blog that the external links provide a way to 'give back' to site builders who provide useful information because the link allows them to improve their online reputation. 

"Although the no-follow move is certainly understandable from a spam-fighting perspective, it turns Wikipedia into something of a black hole on the net. It sucks up vast quantities of link energy but never releases any," he wrote.