An early version of Microsoft's next release of Windows, codenamed Longhorn, may have turned up on the internet.

Appearing to date from late September, the leaked build, numbered 3683, shows off some new conceptual features, many of which are not yet functional.

Commentators believe that the leak is similar to the situation which occurred during the alpha stages of Windows XP, codenamed Whistler at the time.

Microsoft has tried to play down the leak, saying that the build is likely to undergo many revisions before officially entering the public domain.

According to BetaNews, information about the leak appeared last month when screenshots sent to Neowin.net, a website for Windows enthusiasts, apparently showcased the new operating system.

However, those with access to the code chose to keep it quiet rather than risk Microsoft's displeasure, until someone sent a copy to BetaNews.

The build, which other commentators believe to be an in-development Windows XP update, includes slight changes to XP and a new visual style, referred to as 'Plex'.

It has a Longhorn sidebar that uses XML to display customisable tiles such as a clock, virtual desktop manager and internet search. The Start Menu can also be docked within the sidebar.

Microsoft has replaced the standard Display Properties dialogue with an XML-based configuration panel that uses a new API framework codenamed Avalon.

The new operating system is identified as Windows Version 6.0, a long way from Windows XP's 5.1 tag.

A spokeswoman for Microsoft said it was "unfortunate that sometimes these things make their way onto the internet".

She could neither confirm nor deny that the screenshots displayed on Neowin.net were leaked from current development work.

But she said it was unlikely that user interfaces in development versions would be seen in the final release.

"With Windows XP there were six different user interfaces in the development versions. I wouldn't be confident that what was shown would appear in the final versions," she said.

Longhorn is expected to be in the shops early in 2005.