One of the most popular sites on the internet, the Yahoo web directory service, was put out of action for three hours when hackers mounted an electronic assault on it.

The firm said hackers did not penetrate its computer systems but blocked access to them with a flood of fake electronic messages.

The technique, called a denial of service attack, is like repeatedly dialling a telephone number to block all other incoming calls.

It took until late on Monday for technicians to fully restore the service after problems began at about 1345 EST (1845 GMT).

Yahoo spokeswoman Diane Hunt said: "Our first priority has been identifying what was happening and then installing the filters to enable our users to access our services."

Analysts say the fact that such a well-established company could be so vulnerable to sabotage is a sobering reminder of the unknown hazards facing internet-based businesses.

Elias Levy, chief technology officer at California-based SecurityFocus.Com, said: "It basically says nobody is safe, if Yahoo can be taken down with all the resources behind them."

Management problem

Another expert said the failure suggested Yahoo was unprepared for such an attack on its systems.

The Yahoo site is among the most robust on the internet, delivering an average of 465m web pages to consumers each day.

"It's kind of silly it took so long," said James Atkinson, president and senior engineer at Granite Island Group, an internet security consultant in Massachusetts.

"The fact it went on for hours indicates a management and infrastructure problem that does not involve technology. This should have taken them off the map for 15 to 20 minutes, 30 at the most."

The FBI suffered a similar attack on its website in April last year.

The site was inaccessible for days after hackers overwhelmed its computers, also using a denial of service attack.

The latest problem also prevented Yahoo customers from accessing free e-mail accounts through the website, but it did not otherwise affect the flow of mail.

Media Metrix, which tracks visitors to websites, says Yahoo was the most popular internet site for the month of December, the latest figures available, when it recorded 36m visitors.