Internet surfers were unable to access sites run by software giant Microsoft for the third successive day on Thursday.

The company said it had been the subject of a "denial-of-service" attack, in which hackers flood systems with so much data that other efforts to access a site are slowed or fail altogether.

Microsoft restored service on Thursday and notified the Federal Bureau of Investigation of the attack, company sources said.

Microsoft shares fell during the shutdown, closing at $61.81 after reaching a high of $64 in the morning.

Earlier shutdown

The attack was unrelated to the human error that had shut down Microsoft sites on Tuesday and Wednesday.

A technician was responsible for that 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.

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 had in October suffered the embarrassment of a hacker stealing part of the code for products in development from its corporate computer network.

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

Along with Yahoo and CNN, the two sites were also crippled for hours at a time by similar attacks last February.