Via its official blog, Google has announced it is to launch a strong rival to the world's free online encyclopedia, Wikipedia.org.

The new service from Google, to be called "knol" - which apparently stands for a unit of knowledge - is to be a free tool available to everyone.

The blog states: "Our goal is to encourage people who know a particular subject to write an authoritative article about it".

Google will provide "easy-to-use" tools for writing and editing and will obviously host the content.

The knols will include "strong community tools" and people will be able to submit comments, questions, edits, additional content.

The knols will appear in Google search results, and anyone will be able to rate or review someone else's knol.

In stark difference to Wikipedia's structure, authors will not only be credited for the work, but can also earn "substantial" advertising revenue if they chose to allow ads to appear on the pages.

Google says the tool is still in development and this is just the first phase of testing via invitation only.