Google is going head-to head with Wikipedia, the Web's largest reference work, setting up a clash between two of the Web's biggest brands.

A new Google service, dubbed "Knol," will invite individuals to write "authoritative articles" on their areas of expertise, the firm revealed Thursday in a blog posting by Vice President of Engineering Udi Manber.

• Click here to read Manber's post on the official Google blog.

As on Wikipedia, content on Knol (the name comes from "knowledge") will be free to access. In a departure from the non-profit Wikipedia model, however, Knol's authors will be able to attach advertising to their work and take a share of revenues.

Google hopes that Knol articles will cover "all topics, from scientific concepts ... to entertainment." Significantly, the project will see Google help generate new editorial content, a process its executive have previously said it is "philosophically opposed to."

Knol, which is currently in a test phase but is expected to be opened to the public in the coming months, also pitches Google against yet another new rival in a fresh sector.

Moving away from its roots in Internet search, Google recently opened a new front against mobile makers such as Nokia by unveiling a new operating system for handheld devices.

It also has ambitions to compete with groups such as EDS in data storage and Oracle and Microsoft in business software.

Last month it confirmed it will bid against groups likely to include AT&T for a portion of America's airwaves that could be used to roll out a wireless broadband network.

Wikipedia represents another similarly well-established incumbent.

In October the online encyclopedia, which relies on donations for funds, was visited by 107 million people, or a third of the "active global Internet population", according to Nielsen Online, the analyst. That made it the eighth most-visited online destination.

Google's search engine was the world's most popular site, with more than 260 million users -- though its own reference work, Google Scholar was only fifteenth in its class, with about 4.5 million users.

Jimmy Wales, the Wikipedia founder who recently launched a rival search engine to Google's, questioned whether Knol would be able to generate enough "quality content". He also suggested that Knol articles would lack balance.

"They are not going to allow collaboration and aren't going to go for Wikipedia's neutral style," he said.

However, Google's determination that Knol should turn a profit appears to have dictated a sharp departure from Wikipedia's editorial model. Where Wikipedia is based on collaboration between authors, Knol will foster rivalry.

Contributors to Knol will not be able to contribute anonymously and will not be able to edit each others' work -- two of the defining characteristics of Wikipedia.

Whereas in Wikipedia, readers find only one entry on, say, the First World War, on Knol authors will submit separate pieces that will compete for advertising income.

Taking a cue from social networking-style sites, Knol will also invite readers to participate by rating the quality of entries and by adding "comments, questions, edits, additional content."

Rebecca Jennings, an analyst for Forrester, said: "Google is setting out to compete in social media, where it is lagging [behind] rivals such as Facebook."

"A Knol ... is meant to be the first thing someone who searches for this topic for the first time will want to read," Manber said in the Google blog posting.

Wikipedia was founded in 2001 and now has more than eight million articles in 253 languages from Afrikaans to Zazaki. In contrast to Google, it has refused to alter its policies to operate in different countries, something that has led it to being blocked in states such as China.

However, anonymous and sometimes malicious edits have threatened to undermine Wikipedia's reputation.

In 2005, in what Wales termed "the worst" incident to hit the site, John Seigenthaler, Sr., the founding editorial director of USA Today, was linked to the assassination of President John F. Kennedy by a Wikipedia article.

Attacking Wikipedia, Seigenthaler called it an irresponsible haven for "volunteer vandals with poison-pen intellects."

This summer, it emerged that a host of blue-chip companies had altered their entries on Wikipedia in an attempt to cover up embarrassing episodes in their histories.

The discovery was made by WikiScanner, a site that traces the source of changes to the world's largest online reference work by matching edits to a database of the unique IP addresses of the computers that were used to make them.

Machines belonging to organizations including Wal-Mart, Disney, Sony, the British Labor Party, the CIA and the Vatican had been used to rewrite entries, it was found.