Computer experts at the world's most prominent software firm have discovered that a technician was responsible for the failure of the company's websites.

In a company press release, Microsoft said that a technician's error "limited communication" between internet servers and the company's own servers.

Between Tuesday afternoon and Wednesday surfers across the world were unable to access addresses including the firm's corporate site, and the e-mail system hotmail.com, which has more than 74 million users worldwide.

At first, the company admitted that it was "mystified" by the problem.

The firm is now ruling out an attack by hackers, or a so-called "denial of service", in which a site is flooded by traffic, rendering it essentially inoperable.

Technical blunder

Late on Tuesday, the Microsoft technician made a configuration change to routers on the edge of the company's Domain Name Server (DNS) network.

The DNS servers connect domain names with the various servers and networks that form Microsoft's presence on the Internet.

The mistaken configuration restricted communication between the different servers and prevented a large number of surfers from reaching many Microsoft websites.

Access denied

Users could not gain access to sites including the Expedia travel service and Encarta encyclopaedia.

The sites affected were operational again by about 0130 GMT on Thursday.

Microsoft's corporate computer network has already suffered an embarrassing problem when a hacker stole part of the code for products in development in October.

Other sites that have also crashed this month include the auction site eBay and the biggest online retailer Amazon.com.