Barclays Bank is to start selling data on its millions of current and savings account customers to other companies for the first time.
The bank has told savers that it intends to package together "information about the transactions on your account" with data on groups of other customers to compile reports on spending trends across Britain. This could then be passed on other companies or Government departments.
Under the changes, which take effect in the autumn, the bank will also start tracking customers through their mobile phones or other "devices" - to help protect them from fraud. If a payment is made in a certain country, Barclays (LSE: BARC.L - news) will "ping" the customer's mobile number to check if the phone is in that country.
The move by Barclays has been laid out in changes to terms and conditions, sent to customers around the country, that take effect from the start of October this year.
It forms part of a growing trend for companies who have realised they can generate new revenue streams by selling data on their customers.
Just days ago, the biggest mobile phone operators in the country, Vodafone (LSE: VOD.L - news) , EE and O2, said they would start selling bundles of anonymised data on their customers to big advertisers to help them target different age groups and demographics.
But Barclays said that any customer data it sells would never be sold on in such a way that an individual customers was "identifiable". A spokeswoman said the data collected would be "high level" numerical information, not personal.
She added the bank believed customers would benefit from the information it collects, such as informing them if they were spending more than the rest of the neighbourhood on their energy bills.
Tesco (Other OTC: TSCDY - news) became the first company to spot the potential for customer data when it launched its Clubcard loyalty card in the Nineties. Data collected through the scheme was packaged together and sold to big food and drink companies so they could plan promotions or new campaigns.
In May, Barclaycard launched a Groupon (NasdaqGS: GRPN - news) -style service , where customers signed up to receive vouchers from the likes of Shell (LSE: RDSB.L - news) and British Airways based around their spending patterns.
Barclays' new terms and conditions will cover all of their 15 million current and savings account holders. Customers will only be able to opt out of having their mobile phone tracked.
In a statement, the bank said: "We only use information in a numerical, anonymised and aggregated way as is standard practice at many companies.
"Mobile location data will be used for fraud prevention purposes only and therefore only on the occasions where this is a transaction on a customer's account that has been picked up by our fraud detection systems.