Eight members of a gang suspected of targeting a branch of Barclays bank in Swiss Cottage, north London, in April, were arrested, and police discovered a control room where they believe the gang masterminded cyber crime on a scale not seen before in Britain. It is believed one of the plotters posed as an engineer and planted a "keyboard video mouse" that allowed thieves to access accounts. The same method was used unsuccessfully at a branch of Santander in east London earlier this month, and both plots are being investigated as part of the same police operation.
Detective superintendent Terry Wilson said: "We believe that of the eight people arrested, they all had specific roles within the criminal network of varying degrees and various levels.
"One of them we consider to be a significant arrest. I would class him probably as the Mr Big of UK cyber crime at present.
"We consider that we have significantly dented this criminal network. We believe that we've now impacted on the top tier."
The men, aged between 24 and 47, were held on suspicion of conspiracy to steal from Barclays bank, and conspiracy to defraud UK banks. Police carried out a secret intelligence operation to target the network.
Mr Wilson said: "We've actually been quite astounded by what we've come across, which effectively is a cyber crime control room in Seymour Street in Marylebone. We have recovered an overwhelming amount of evidence, so much so that we've had to draft in further resources."
The "logistical control room" was in a residential address and allowed cyber crime "on a scale that we've never really seen before", he added.
Searches have been carried out at various addresses in London and Essex, where officers have discovered "a huge amount of evidence", including cash, jewellery, drugs, thousands of credit cards and personal data.
Mr Wilson would not comment on whether other banks had suffered similar attempted thefts, but said none had lost money.
Customers at the Swiss Cottage branch of Barclays expressed their concern at news of the theft, although the bank has confirmed that no customers lost money as a result.