The insiders scoured the computer system for dormant accounts holding large sums of money and passed the details to the criminal gang.
They also ordered new bank cards and altered phone numbers and addresses so that imposters could pose as the customers and set up transfers of hundreds of thousands of pounds at a time.
The money was then laundered through a series of bogus companies before being moved offshore to prevent it being recovered.
One victim lost more than £750,000 after an unknown man used a fake driving licence in his name to set up two transfers over three days.
Another customer with £3m in the bank only escaped becoming a victim when another member of staff became suspicious and rang him up to check.
Bank workers Courtney Ayinbode, 29, Tajinder Galsinh, 35, Molly Jones, 24, and Benjamin Omoregie, 26, were all involved in the 'high-level and sophisticated' scam.
The money was washed by 'bespoke money laundered' Eddie Lakes, 41, and his henchman Kushveer Raulia, 25, through a complicated network of fake companies and accounts with the help of Parvez Hussain, 50.
Prosecutor Paul Cavin said “a lot of expensive designer clothing and shoes” were found in Omoregie's car boot, including a Vivienne Westwood shirt worth £230.
Omoregie had previously refused to tell police where his car, holding the stash of designer goods, was located.
The prosecutor said the gang “were all involved in an agreement to defraud Lloyds TSB of millions of pounds”.
He continued: “Accounts belonging to people with large balances - in some cases several millions of pounds - were targeted.
“To identify those rich accounts, an insider, an employee of the bank was recruited to help the gang.
“The insider would then assist in breaching the security protocols in order to steal the money.
“The money would be transferred to accomplices who would swiftly transfer the money through a number of bogus businesses before eventually the money would disappear offshore.
“Once it is offshore it can come back onshore and nobody can trace it.”
The first victim lost £750,975 in the space of three days after his account was accessed by both Ayinbode and Omoregie at the Balham and Streatham branches of Lloyds TSB in July 2013.
A few days later the customer's address was changed and a new card was issued to the new address.
Then on 5 August a man claiming to be the customer went into the Kings Cross branch to ask to set up a transfer.