Three of the criminals were bank staff, who between August and October 2007 targeted the accounts of wealthy customers who used the Sloane Square, London branch of Barclays Bank.
Three of the attempts were thwarted, but on five occasions the fraudsters succeeded, stealing a total of more than half a million from the bank.
Cashiers Mohammed Ashif Iqbal, 30 and Tommy Lee Jackman, 24, worked at the London branch.
A third man, Chikezie Emeruem, 33, from Leeds, worked at the Market Street, Bradford branch of the bank.
The proceeds from two of the fraudulent transactions were credited to the business accounts of Karl Hession, 29, and Naresh Patwari, 50.
Iqbal was sentenced to five years in prison, Jackman to four years six months, Emeruem to three years six months, Patwari to two years and Hession to one year at Southwark Crown Court last Thursday.
The men used their security clearance to check whether accounts were worth stealing from.
Customer contact details were altered so that legitimate customers could not be alerted to large transactions on their accounts by other bank staff conducting security checks.
Iqbal, from Stanmore in Middlesex, and Jackman, from Enfield, North London, also provided fake identity documents in the names of wealthy bank customers.
When Jackman was arrested in December 2007 he had moved from Barclays to a new job at Abbey. Officers who searched his home found compromised customer details relating to ten different Abbey accounts.
All five men had denied being part of a fraud but were convicted earlier this month following a six-week trial at Southwark Crown Court.
Detective inspector Perry Stokes says: “This has been a complex and lengthy investigation into a very determined and sophisticated criminal gang whose sole intent was to steal large amounts of money from the customers of Barclays.
“The former Barclays employees Iqbal, Jackman and Emeruem were key to the success of this fraud. Their arrogant behaviour throughout the period of offending and the judicial process has displayed a real contempt for the law, with no signs remorse for their actions.
“They didn’t bat an eyelid at compromising the personal account details of the very customers they were supposed to be safeguarding and were happy to try to lay the blame for their crimes on innocent colleagues within the bank.”