The competition for the title of "best search engine" is as tight as possible, and each of the three main rivals, Google, Yahoo! and AskJeeves, are trying to extend their services as far as possible, by including in the search results an ever-increasing quantity of information.

In order to support these efforts, Yahoo Inc. has announced on Wednesday that it has begun testing a service for searching information on subscription sites, also known as the "deep Web."

The new service, called Yahoo! Search Subscriptions, which can be accessed at http://www.search.yahoo.com/subscriptions allows the users to view the list of subscription-based sources, the subscribers being offered the possibility to view their content, while the other users can chose to pay the fee for the same option.

At the moment, the sources available through Yahoo! Search Subscriptions are ConsumerReports.org, The Wall Street Journal Online, The New England Journal of Medicine and Forrester Research Inc, but it seems that over the next few weeks, Factiva, LexisNexis and Thomson Gale will also join the club.

Yahoo hasn't yet established revenue sharing relationships with any of the content providers, but it's been acquiring content for over a year now, and will add more sources to its service as it develops, said David Mandelbrot, Yahoo's vice president of search content.