Internet users are being warned about a strain of malware which has allowed criminals to steal £20m from UK bank accounts.
Dridex malware, also known as Bugat and Cridex, has been developed by cyber crooks in Eastern Europe to harvest online banking details, which are then used to steal cash from individuals and businesses around the world.
Computers become infected when users receive and open documents in seemingly legitimate emails.
The National Crime Agency believes there could be thousands of infected computers in the UK, the majority being Windows users.
The agency is working with the FBI to ‘sinkhole’ the malware, to stop infected computers - known as a botnet - from communicating with the cyber criminals controlling them.
It said a large part of the botnet had been made harmless and that action was being taken to safeguard victims.
An international effort involving Europol, GCHQ and the Moldovan authorities was under way to disrupt the sophisticated cyber criminal network, resulting in a "significant arrest" with more expected.
The NCA is encouraging all internet users to ensure they have up-to-date operating systems and anti-virus software installed on their machines, to protect themselves from further cyber crime attacks.
Anyone who thinks they may have lost money as a result of malware were advised to alert their bank and contact the cyber crime reporting centre Action Fraud.
Mike Hulett, head of operations at the NCA’s National Cyber Crime Unit (NCCU) said: "This is a particularly virulent form of malware and we have been working with our international law enforcement partners, as well as key partners from industry, to mitigate the damage it causes.
"Our investigation is ongoing and we expect further arrests to made."
The FBI's executive assistant director Robert Anderson said: "Those who commit cyber crime are very often highly-skilled and can be operating from different countries and continents.
"They can and will deploy new malware and we, along with our partners, are alive to this threat and are constantly devising new approaches to tackle cyber crime."