According to the Daily Mail, the banking giant notified customers of a change to its terms in a letter sent out last week. Only those who read the small print will have discovered that the bank may use information obtained from accounts, as well as "location data derived from any mobile device details you have given us", to pass on to other firms, government departments or MPs.
Barclays advised: "We can combine information about you with information about other Barclays customers to create reports which we may share with companies outside Barclays."
The letter also explained that the bank "can use information about the transactions on your account to increase our understanding of services and products that you may wish to use".
While Barclays customers can opt out of the latter part of the changes, the Mail reports that some 13.2 million account holders will be affected by these plans to share data, and the only way to prevent it doing so completely is to close the account.
But Nick Pickles, director of privacy campaign group Big Brother Watch, said Barclays was simply using the customer as a "cash cow".
He told the Mail: "This is yet more evidence of an arms race between big companies who are intent on selling out their customers' privacy for their own profit.
"Barclays is following in the footsteps of a host of companies who no longer see their customers as people to provide a service to but instead a cash cow for personal data, to be monitored and sold on to advertisers with little regard for asking people's permission."
In response, Barclays said the information was used in a "numerical, anonymised and aggregated way", and that "personal data will never be passed on to anybody else without their explicit consent".
HSBC, RBS and Lloyds Banking Group denied using the practice.
What do you think? Would you be happy for your bank to use data in such a way? Leave your comments below...