Twelve men have been arrested over an alleged "audacious" plot to remotely take control of a bank's computer.
Police found a device fitted to a computer in a branch of Santander which would have enabled the suspects to download data from the desktop machine.
Police arrested 12 men, aged between 23 and 50, in London on suspicion of conspiracy to steal.
The "very significant" plot targeted the bank branch in Surrey Quays in south-east London, police said.
Several addresses in Hounslow, Brent, Hillingdon, Westminster, Richmond and Slough were searched and property was seized.
'Remote takeover'The arrested men, who were detained on Thursday, remain in police custody.
The Metropolitan Police said its "time-critical, dynamic response" had thwarted a "very significant and audacious cyber-enabled offence".
The force said the operation had helped avoid "multimillion-pound losses from Santander at Surrey Quays shopping centre".
"The offence involved deploying a KVM (keyboard video mouse) device, fitted to a computer within the bank branch, allowing the transmission of the complete desktop contents of the bank computer over the network," a Met Police spokesperson said.
This allowed the remote takeover of the bank's computers, he added.