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 heist took place over the past 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 $1bn (£648m) was taken in the attack and the enterprise, called Carbanak, is still active.
Commenting on the news, Professor Mike Jackson, a cyber security expert from Birmingham City University, said:
This hacking attack appears to have exploited the weakest cyber link in any organisation – the employees.
The act of opening an email and clicking on a link, which can appear so innocuous has resulted in major losses for a number of financial establishments.
Prof Mike Jackson
The gang used malware to infect organisations’ networks - by sending official-looking emails that employees would click on and thus enable the hackers to see and record everything happening on the employees’ screen.
The largest sums were grabbed by hacking into banks and stealing up to $10m per raid, it is estimated.
A number of UK financial organisations were attacked but data does not show whether the attempts succeeded, and not all financial institutions reported the incidents.
Sergey Golovanov, a researcher for Kaspersky Lab, said: “It was a very slick and professional cyber-robbery.”