Microsoft's grip of the key internet browser market is slipping as consumers opt for opensource software, according to a survey.

The decline prompted the software giant to contact clients, urging them to think twice before switching to free alternatives to its own products.

OneStat, the Netherlands-based web analytics consultants, said that Microsoft's Internet Explorer continued to lead the browser field but had slipped by 5 per cent from May to account for 89 per cent of browser use this month.

The main beneficiary was the Mozilla Foundation, a non-profit organisation that develops software which it makes available to download for free on the internet at www.mozilla.org.

Niels Brinkman, co-founder of OneStat.com, said: "It seems that people are switching from Microsoft's Internet Explorer to Mozilla's new Firefox browser. The total usage share of Microsoft declined 5 percent and the total usage share of Mozilla increased 5 per cent."

The news came as Mozilla recorded 5 million downloads of its new Firefox browser. Firefox 1.0, a descendent of Netscape, the former market leader, was released earlier this month. Earlier versions of the software had gained a cult following by supporters who have claimed it is more reliable than Microsoft's.

Such has been the software's appeal, users banded together to raise $250,000 and advertised this month's release of Firefox 1.0 in The New York Times.

Asa Dotzler, Mozilla's release manager, said: "We're seeing a much swifter uptake for 1.0 than for the preview release, which took more than a month to reach 5 million downloads. We're clearly reaching a new world of users and we're doing it at a faster pace than any time in Mozilla's history."

OneStat said that Mozilla's browsers has a total global usage share of 7.35 per cent in November. The new Mozilla's Firefox has a total usage share of 4.58. The total usage share of Mozilla was 2.1 per cent at the end of May.

In an indication of how seriously Microsoft are treating the threat posed by free opensource software Steve Ballmer, Microsoft's chief executive, today wrote a letter to clients hoping to dissuade users from switching to opensource, Unix and Linux systems.

Mr Ballmer quoted a number of surveys including "an independent, non-sponsored global study of 1,000 IT administrators and executives" conducted by Yankee Group, the consultants.

"Yankee's study concluded that, in large enterprises, a significant Linux deployment or total switch from Windows to Linux would be three to four times more expensive – and take three times as long to deploy – as an upgrade from one version of Windows to a newer release," Mr Ballmer said.

"Nine out of 10 enterprise customers said that such a change wouldn't provide any tangible business gains," he added.

Earlier this month Adam Dawson, of SecurityLearning, the internet security company, told Times Online: "Alternative web browsers like Opera, Firefox and, in the UK, Deepnet, are exploiting perceived weak spots in Internet Explorer to gain the hearts and minds of worldwide surfers.

"Users are tired of the security threats and frustrated by the patch-up tactics that have kept Explorer on the superhighway for more than two years without a major service."

OneStat's research was based on a sample of 2 million visitors divided into 20,000 visitors of 100 countries each day.

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