Seven people are jointly accused of being involved in an identity theft plot, to steal more than £1.3m of bank customers' money.
Five men, all from Berkshire and London, and one woman, deny conspiracy to defraud and launder money.
A London court heard how confidential details of "high value accounts" were given out between 2004 and 2005.
The money was taken and laundered through a string of car dealerships, Southwark Crown Court was told.
Emmanuel Imbrah, 21, of Reading, Neil Kynoch, 38, of Windsor, Dominic Almond, 39, of Slough, Shana Campbell, 23, of Brockley, London, and Steven Fabian, 44, of Cannington, London, and Ayodeji Osibogun, all deny the charges.
Olawasegun Adekunle, 27, of South Lambeth, London, admitted his role in the plot.
The court heard how he played a "central" role and led a "life of luxury at the expense of others" as a result of the operation, which involved a number of other people who have not been identified.
The court heard how the group used false passports and utility bills, created through confidential information smuggled from banks, to obtain the money.
Francis Sheridan, prosecuting, said: "The defendants and other persons unknown embarked upon a highly organised and systematic operation to siphon the funds out of the legitimate accounts into new accounts that they had set up by using either telephone banking and or the internet."
He added the new accounts were set up in the names of genuine customers before the money was laundered.
Customers of the Halifax Bank of Scotland and HSBC banks lost £785,712 and £549,726 in the plot respectively.
Mr Sheridan said some Barclays customers had also been "fleeced" by the gang.
He added Campbell and Imbrah were two of a number of bank insiders involved.
Mr Sheridan told the court: "[Campbell] was using all the knowledge she had gained in her employment to the disadvantage of her employers and their customers."
The trial continues.