YouTube, the video-sharing site owned by Google, is set to topple the BBC's flagship website as the most popular online media destination in the UK.

According to Hitwise, the market researchers, bbc.co.uk has been the most-visited "entertainment" website in the UK every week for the past two years but is set to be passed by YouTube "in a matter of weeks".

The impending demotion of bbc.co.uk to second place at the hands of YouTube looks to underscore the power of “web 2.0” media models, which harness user-created content and strive to create online communities.

The continued rise of YouTube, which downloads more than 100 million video clips a day, could also vindicate Google's decision to buy it for $1.6 billion last year despite the site being yet to turn a profit and significant legal concerns over pirated content.

Earlier this year, Viacom, the media giant, sued YouTube for $1 billion (£502 million) over bootlegged video clips.

However, the Hitwise figures do not take account of BBC sites that do not fall under the main "bbc.co.uk" domain – which includes pages from the broadcaster's news, sports and weather services which take addresses that start "news.bbc.co.uk".

According to Nielsen//NetRatings, another online research group, when the entire family of BBC websites is included, the corporation still outstrips YouTube by a large margin, attracting about 16 million users in April in the UK – about twice the figure achieved by YouTube.

Even so, Nielsen also suggests that YouTube is catching the BBC fast. According to the researcher, the BBC's entire online audience grew by 6 per cent in the six months to April, compared to nearly 50 per cent growth for YouTube.

According to Hitwise, last week, bbc.co.uk was the fourteenth most popular site in the UK, with 0.82 per cent of all UK internet visits.

YouTube, which has been the second most popular media site since October, came in at 25 in the overall rankings, with 0.81 per cent of UK internet visits.

However, the gap between the two sites is closing rapidly. Last week, BBC.co.uk had a 1.6 per cent lead on YouTube in terms of share of UK visits, down from 37 per cent three months ago and 131 per cent six months ago.

The share of UK visits to bbc.co.uk has continued to grow, up 13 per cent year on year in May. But that pace has been outstripped by the increase in traffic for YouTube, whose share of UK visits was up nearly seven-fold year on year in May and up 140 per cent in the past six months.

Hitwise collects data from a range of internet service providers on the number of website visits served through their networks. The figures are used to calculate a website's position in the rankings.

YouTube could receive a further boost from the news that its content will be made available on Apple's hotly-awaited iPhone, which will be launched in the US at the end of this month. Hoping to extend its popularity to handheld gadgets, the video site has begun encoding its clips in a new format to improve quality and save battery life when viewed over wireless devices.

Earlier this week, YouTube unveiled nine country-specific sites, including a UK site, with customised search, local language interfaces, and charts showing the most popular domestic content.

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