Search giant Google has dropped to its lowest percentage of the UK market share in five years, Experian Hitwise, a firm that monitors web traffic, says.
October figures released by the company suggest 89.33% of all web searches in the UK were made using Google.
Its main rival, Microsoft's Bing, now has 5% of the market share, with Yahoo in third place.
Microsoft's recently launched Windows 8 operating system has Bing installed by default instead of Google.
Analyst Luca Paderni, from Forrester Research, said: "In the UK, Bing has been using very aggressive tactics of promotion for last few months, in preparation for the Christmas season.
"But Google is still dominant, and we would need to see a trend over more months to call it a consistent decline."
Although Google is the leading search engine in many places around the world, alternative search engines have managed to get ahead of it in their home markets.
In China, Baidu is number one, and in Russia, the leader is Yandex, with Google in second place.
In Europe, Google's competitors would need to concentrate on services other than general purpose search to improve their chances of rivalling the search giant, said Mr Paderni.
"There's increasingly more space for search services specialising in specific industries or topics," he said.
Hitwise told BBC News it had used data from various internet service providers in the UK to track the search habits of eight million people.
The firm's digital insight manager said web users were "demanding more of the engines they interact with everyday".
"The search engines that remain reliable, relevant and useful will be the ones that profit in the future."
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