Internet around the world has been slowed down in what security experts are describing as the biggest cyber-attack of its kind in history.
A row between a spam-fighting group and hosting firm has sparked retaliation attacks affecting the wider internet.
It is having an impact on popular services like Netflix – and experts worry it could escalate to affect banking and email systems.
Five national cyber-police-forces are investigating the attacks.
Spamhaus, a group based in both London and Geneva, is a non-profit organisation that aims to help email providers filter out spam and other unwanted content.
To do this, the group maintains a number of blocklists – a database of servers known to be being used for malicious purposes.
Recently, Spamhaus blocked servers maintained by Cyberbunker, a Dutch web host that states it will host anything with the exception of child pornography or terrorism-related material.
Sven Olaf Kamphuis, who claims to be a spokesman for Cyberbunker, said, in a message, that Spamhaus was abusing its position, and should not be allowed to decide “what goes and does not go on the internet”.
Spamhaus has alleged that Cyberbunker, in cooperation with “criminal gangs” from Eastern Europe and Russia, is behind the attack. Cyberbunker has not responded to the BBC‘s request for comment.
Steve Linford, chief executive for Spamhaus, told the BBC the scale of the attack was unprecedented. “We’ve been under this cyber-attack for well over a week”.
Writing exactly one year ago for the BBC, Prof Alan Woodward predicted the inherent weaknesses in the web’s domain name system.
He wrote: “It is essentially the phone book for the internet. If you could prevent access to the phone book then you would effectively render the web useless.”
“But we’re up – they haven’t been able to knock us down. Our engineers are doing an immense job in keeping it up – this sort of attack would take down pretty much anything else.”
Mr Linford told the BBC that the attack was being investigated by five different national cyber-police-forces around the world.
He claimed he was unable to disclose more details because the forces were concerned that they too may suffer attacks on their own infrastructure.
The attackers have used a tactic known as Distributed Denial of Service (DDoS), which floods the intended target with large amounts of traffic in an attempt to render it unreachable.
In this case, Spamhaus’s Domain Name System (DNS) servers were targeted – the infrastructure that joins domain names, such as bbc.co.uk, the website’s numerical internet protocol address.
Mr Linford said the attack’s power would be strong enough to take down government internet infrastructure.