According to data from comScore Media Metrix, Google had a 56.5% share of the US search market, compared to Yahoo wtih 23.3% and Microsoft with 11.3%. In the UK, Google's dominance is even greater with around 75% market share.

Its dominance has forced rivals to relaunch, add new features and improve the relevance of its searches in an effort to compete.

Microsoft's improved Live Search - which will be available globally by the end of October - will offer a series of new featues, including a quadrupling of its index of searchable web pages as well as improving the way it delivers answers to specific questions.

It will draw on its three-dimensional mapping service for location-relevant searches as well as offering video, charts and pictures for relevant results.

Yahoo's upgrades include offering ways to phrase a search request as a user types into the query box. It will also use information from its calendar service, Upcoming.org, to highlight local events when relevant to requests.

Search rival Ask.com relaunched with a dramatically different interface in June, offering search suggestions and thumbnail pictures of listed sites.

There has been a rise in so-called metasearch engines which trawl through a variety of other search engines to find results.