The money was stolen from the bank's Swiss Cottage branch in north London in April using a device known as a keyboard- video-and-mouse (KVM) switch.
"The arrests are the result of a long term intelligence led operation by the Metropolitan Police's Central e-Crime Unit (PeCU), in partnership with Barclays Bank, who have been investigating the theft of 1.3 million pounds from the Swiss Cottage branch of Barclays in April 2013," the Met Police said in a statement here today.
Their identities have not been disclosed as yet.
The operation follows charges last week against four others, including a 27-year-old Indian-origin man Akash Vaghela from west London who is out on bail and will appear on conspiracy to steal charges at London's Southwark Crown Court on September 27.
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He was arrested earlier this month alongside three others over an aborted plot to take control of a computer at a south-east London branch of Santander bank, also using a KVM device.
The attempt to steal from Santander last week is being linked to the theft at the Barclays, BBC reported.
The hardware for the operation, which can be purchased online for as little as 10 pounds and is the size of a small laptop computer, allowed the gang to transmit the contents of the computer's desktop and take control of the machine remotely.
It is believed the device was installed by a man who pretended to be an IT engineer to gain access to the branch and attached the KVM device with a 3G router to one of the computers, which allowed the gang to remotely transfer money into their bank accounts.