Google hopes to knock Wikipedia off its perch as the pre-eminent "community encyclopaedia" by releasing its own compendium of articles about every possible topic.

The search giant today released Knol, a service which encourages any user to write articles as an "expert" on any subject - from fly-fishing to Byzantine architecture or even how to install a sink.

There is no vetting process, although readers can rate articles - as they can rank stories on the "social news" service Digg. Authors can also elect to have advertising on their articles, and share in the revenue from the adverts, which will be placed by Google's ad-serving platform, AdSense.

Unlike Wikipedia, readers will not be free to edit articles, though the ranking system will mean that those 'knols' which are popular will fare better in searches conducted on the Knol home page. Knol entries will also appear in regular Google searches, but their ranking will be determined in the usual way - according to how many other web pages link to them.

A source close to Google said the service was not designed to be a Wikipedia-killer. A better way to think of it would be as a platform for pages more like blogs, the source said, but instead of being updated all the time, the entries were a chance for publishers to share their expertise.

Internet experts said the service was unlikely to compete with Wikipedia - which now contains more than seven million articles in 200 languages - because it would be a platform for opinion, rather than factual content.

Whereas on Wikipedia, multiple anonymous writers help to create a single entry on dinosaurs, say, on Knol, several "dinosaur experts" could each contribute an entry and vie to create the most popular dinosaur page, entitling them to a greater share of dinosaur-related advertising revenue.

Knol is, however, likely to attract some users from similar services such as Squidoo, which has the slogan "Everyone's an expert on something" and is visited by six million people a month, and Videojug, a similarly themed service based on video content, which has two million visitors.

"There's a lot of competition in this area, because there's generally a lot of interest in creating content - we've seen that with blogging," Rebecca Jennings, an analyst with Forrester, said. "Knol is unlikely to be a Wikipedia killer, though, because it uses very different tools, but it will appeal to people who are passionate about a topic and want to get their opinions out there."

Ms Jennings added that the service may ultimately be of greater interest to publishers keen to make money from their articles, because they would not be forced to pay what was effectively a "middle man" fee to competing share-your-expertise sites, many of which rely on Google's Ad Sense program to place their adverts.

Wikipedia, which has more than 250 million visitors a month, according to comScore, is run by a charity and has never accepted advertising.

Wikipedia has grown rapidly to become the most popular encyclopaedia on the web, and is often cited as a reference, but of late it has been challenged by those who oppose the growing power of "amateur experts".

In September last year, one of Wikipedia's co-founders, Larry Sanger launched a rival service called Citizendium, which will solicit material from the public but will be edited by experts.

The idea that "open collaborative projects" could replace the work of professional individuals was "an extraordinary popular delusion," one of Citizendium's backers said.

-Separately, Google was today reported to be in final negotiations to buy Digg, the social news service, for about $200 million.

According to a report on TechCrunch, the influential technology blog, the companies have reportedly signed a letter of intent that would make Digg part of Google News, a service that aggregates various news sources by topic area.

Digg, a Silicon Valley company, helped pioneer a social ranking system for news, where stories achieved higher prominence on the Digg site the more people voted for them.

Google declined to comment on the report.

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