War of the web: Yahoo and Google search for winner

Yahoo said yesterday that its search technology now covers 20 billion online documents and images, nearly double the material indexed by Google.

But Google fired back immediately, accusing Yahoo of inflating its search statistics by including duplicates of documents. "We welcome innovation in search, but we have not been able to verify a substantial increase to Yahoo's web index via their search results," a company spokesman said.

The spat is the latest twist in the increasingly competitive world of internet search companies. Google has for some time enjoyed a lion's share of the market, and its popularity has even led to the creation of the verb, "to google".

However, other major internet companies are vying for a larger stake. Bill Gates recently promised to pour millions into MSN's search engine, as did executives for IBM's portal.

Yahoo said its expansion has given it a foot up in the competition over which site boasts the most powerful search engine.

"This is a great reason for more people to check us out," said Yahoo's vice president of products, Eckart Walther. "We are more comprehensive than anyone else out there."

The scope of internet search engines, and thus indirectly the size of the internet, has long been a matter of debate. Until Monday, Yahoo had not publicly disclosed the size of its search index, but industry estimates had placed it on a par with Google at between 6 billion and 8 billion documents.

Verifying the index claims of the search engines is virtually impossible because there is no official auditing system, said Danny Sullivan, editor of industry newsletter Search Engine Watch. Industry experts also said Yahoo's expansion does not necessarily mean it produces more useful results than Google - the real litmus test for search engines.

Up to a point, the size of a search index is important - the more documents searched, the more likely the correct page will be found. But once a search engine has access to billions of pages, what becomes more crucial is its ability to locate the desired result. The formulas used to collate and display internet search results are closely-guarded secrets.

Andy Shaw, technical editor at Web User Magazine said: "Having the biggest index doesn't necessarily make you the best search engine. The way that Yahoo chooses which items in its index are of most relevance to a searcher, and the order in which it brings those results up, is far more important. After all, it doesn't matter how many pages of results are delivered, a searcher is only ever going to look at the first few that appear. Quality is far more important than quantity."

Mr Sullivan added: "There have been times when other search indexes have expanded, and the results have actually gotten worse."

The fierce competition between Yahoo and Google began in earnest last year when Google introduced its own search technology, ending a business partnership between the companies. For the previous three-and-a-half years, Yahoo had been licensing its search results through Google - an arrangement that helped turn its rival into one of the internet's biggest success stories.

Since the split, Google has maintained a comfortable lead over Yahoo, even as its challenger continued to roll out new features that impressed industry analysts. Through June, Google held a 36.9 percent share of the US search engine market with Yahoo at 30.4 percent, according to comScore Networks. The European figures are similar, with Google most popular in Britain.

Generating searches is crucial for both companies because the requests spur revenue-producing ads alongside the results. The strategy has proven highly effective, with Google earning $712 million through the first half of the year while Yahoo earned $959 million.