According to M86 Security, web-based malware technology was utilised by criminals to steal client money via the bankâs online system.
The attack, which the online protection experts claimed is currently ongoing, is thought to have seen the criminals embezzle as much as Â£675,000.
Researchers at the firm believe they have located a control centre in Eastern Europe, which is believed to be the base for the cybercriminals.
In a statement, M86 Security said: âThe Trojan, a Zeus v3, steals the customer's online banking ID and hijacks their online banking sessions. It then checks the account balance and, if the account balance is bigger than Â£800 value, it issues a money transfer transaction.â
Meanwhile, a 26 year-old Estonian has been extradited to the US to face allegations of computer fraud and identity theft.
According to the Federal Bureau of Investigation, Sergei Tsurikov was part of a team of hackers who stole $9.4 million from RBS WorldPay, a payments division of the Royal Bank of Scotland.
By Jim Ottewill