Spamhaus, a London and Geneva-based non-profit group which helps weed out unsolicited "spam" messages for email providers, said it had been subjected to "distributed denial of service" (DDoS) attacks on an unprecedented scale for more than a week.
"There may be further disruptions on a larger scale as the attack escalates."
Spamhaus publishes blacklists used by internet service providers (ISPs) to weed out spam in email traffic.
The group is directly or indirectly responsible for filtering as much as 80 percent of daily spam messages, according to Cloudflare, a company that said it was helping Spamhaus mitigate the attack.
"We've been under this cyber-attack for well over a week," Steve Linford, chief executive of Spamhaus, told the BBC. "They are targeting every part of the internet infrastructure that they feel can be brought down."Perpetrators of DDoS attacks typically target websites by flooding servers with messages from multiple systems so they cannot identify and respond to legitimate traffic.
Paul Vlissidis, group technical director at internet security firm NCC, said the volumes of traffic involved in the attack were having a knock-on effect on the rest of the internet.
Because many computers were involved in the attack, it was difficult to defend against.
"If you have a few computers sending large amounts of traffic you can filter them out easily. When literally thousands and thousands are involved it makes it much, much harder," he told Reuters.
However, according to thinkbroadband, an independent British information website which allows users to test their broadband speed, there appeared to be little evidence of a slowdown.
"Of course it is possible that people may be finding some services or sites they access over the Internet are performing slower than usual ... but there appears to be no evidence to say that UK broadband users have been slowed down across the board," it said on its blog.
(Reporting by Paul Sandle, Kate Holton and Michael Holden; Editing by Peter Graff)
