The true size of Yahoo's index has never officially been revealed, although most industry watchers pegged it at between 6 billion and 8 billion, compared to more than 11 billion for number one search engine Google. But Monday's announcement means that Yahoo has the biggest index among the major search engines -- giving Yahoo bragging rights within the industry.

Still, the effectiveness of a search engine is not dictated solely by the size of its index, and most experts agree that Google still churns out the most accurate results, helping it to maintain a lead in terms of user figures globally. For its part, Google has said that it has not been able to verify Yahoo's claims, but stated that it welcomes all innovation in the field of internet searching.

Currently, the two firms, along with MSN, are locked in a fierce battle for dominance of the increasingly lucrative search engine business, which draws revenues mainly from advertisements associated with search engine results, a idea pioneered by Overture Services (now part of Yahoo), and capitalised upon by Google.

On Monday, JupiterResearch released its 2005 Online Advertising Forecast at the Search Engine Strategies Conference & Expo 2005, claiming that in 2010, the global online advertising market will be worth USD18.9 billion, compared to USD9.3 billion at the end of 2004. In five years' time, the keyword search ads segment of the market will generate more revenue and will grow twice as fast as the banner ads segment, which has been the industry's staple ad format for years.

"There is phenomenal momentum behind search engine advertising," stated JupiterResearch senior analyst Gary Stein. "The number of advertisers using search to market products continues to grow, as does the overall efficiency of the market-- search engines are getting even better at making money off search engine results pages."