Internet search engine Yahoo is today due to unveil details of a much-anticipated revamp of its online advertising system.

The California-headquartered company says the changes will make it easier for advertisers to run sophisticated keyword ad campaigns, judge their effectiveness and to improve them.

As part of the overhaul Yahoo will also change the way it prioritizes ads that are displayed in search results.

Presently, the search engine ranks ads based on the amount the advertisers bid to have their message displayed - an approach that can result in an ad holding onto a key position even if no users click on it, thereby reducing Yahoo's 'click through' income.

The new system will go fully live in the fall. It is the first revamp since the platform was launched by internet start-up GoTo in the late-1990s.

Tim Cadogan, Yahoo's vp of search, says it is designed to act as the foundation on which much of the company's advertising business will run for at least the next five years, as it continues its assault on the prime spot currently occupied by rival Google.

Data sourced from BBC Online; additional content by WARC staff