The internet around the world has been slowed down in what security experts are describing as the biggest cyber-attack of its kind in history, the BBC reported Wednesday.

The attack began as a spat between a spam-fighting group and a hosting firm, and has sparked retaliation attacks affecting the wider internet. It is impacting 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, the BBC said.

The anti-spam group, Spamhaus, based in London and Geneva, is a non-profit organization that aims to help email providers filter out spam and other unwanted content. To do this, the group maintains a number of “blocklists” of servers known to be being used for malicious purposes.

Recently, Spamhaus blocked servers maintained by Cyberbunker, a Dutch web host which 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.