Microsoft has limited online access to a test version of its next Windows system for fear of bringing the internet to a halt.

A downloadable file of Vista software, made available freely online last week, is so big that giving users unrestricted access would clog up online networks, according to the company.

"It's not that we didn't anticipate this level of interest or demand, but that we are at the threshold of what the internet can bear, " reads a posting on the weblog run by the development team behind the software.

Microsoft plans to release Windows Vista on new PCs early next year, marking the first new version of the operating system for five years. The company made a version available, in the hope that exhaustive testing by amateur geeks would find bugs that still needed to be fixed.

The standard version of the software takes up 3.5 gigabytes of memory - equivalent to more than 800 digital songs on Apple's iTunes service. A second version, designed for 64-bit machines, runs to 4.4 gigabytes and takes five and a half hours to download even on a fast cable or DSL connection, according to Microsoft.

Several technology bloggers reported an instant message conversation with a Microsoft representative, who was quoted as saying: "We are literally saying that if we increased our bandwidth any further there's a possibility of taking down the internet - people might have problems with World Cup viewing, etc. "

Kieran Taylor, director of product management at Akamai Technologies, which specialises in online distribution, said the internet was unlikely to be shut down completely by any one event like this. However, he added that it could degrade internet quality for a large proportion of web users.