Hackers launched a sustained attack last night against key root servers which form the backbone of the internet.

Security firm Sophos said that botnets of zombie PCs bombarded the internet's domain name system (DNS) servers with traffic.

"These zombie computers could have brought the web to its knees," said Graham Cluley, senior technology consultant at Sophos.

"While the resilience of the root servers should be commended, more needs to be done to tackle the root of the problem: the lax attitude of some users towards IT security."

Cluley explained that root servers, which manage the internet's DNS, help to convert website names to their numeric IP address, essentially acting as an address book for the internet.

Three of the 13 servers at the top of the DNS hierarchy are said to have felt the impact of the attack, although none is thought to have stopped working entirely.

"If the DNS servers were to fall over then pandemonium would ensue, emphasising the importance of properly defending all PCs from being taken over by hackers," said Cluley.

"A denial-of-service attack like this swamps web-connected servers with traffic from many computers around the globe.

"It is a bit like 20 hippos trying to get through a revolving door at the same time: there is no route through and everything clogs up.

"Fortunately the system is designed to be extremely resilient to these kind of attacks, and the average man in the street will not have noticed any impact. "