Cyber-hackers are thought to have stolen hundreds of millions of pounds from up to 100 banks and financial institutions across the world, including the UK.
The massive heist took place over the last two years, and is believed to have been carried out by a gang of criminals from Russia, Ukraine, other parts of Europe, and China.
Computer security firm Kaspersky Lab estimates that one billion US dollars or £648 million was taken in the attack and the enterprise, called Carbanak, is still active.
Kaspersky said the "malicious" gang's plot marked the beginning of a new stage in the evolution of cyber-criminal activity, where users steal money directly from banks and avoid targeting end-users.
The gang used malware to infect organisations' networks, enabling them to see and record everything happening on employees' screens.
The largest sums were grabbed by hacking into banks and stealing up to ten million dollars in each raid, it is estimated.
The global theft is being investigated by Interpol and Europol.