In a change to its terms and conditions, the bank says it will be able to combine information about customers with that of others to create reports that "we may share with companies outside Barclays".

However, it insists that "information is numerical and not personal, and you will never be identifiable on the basis of it".

Selling anonymised data to outside players such as advertisers is an increasingly common practice, as firms try to monetise ever-growing mountains of big data. The Barclays move comes days after telcos Vodafone, EE and O2 outlined similar plans.

But it is still a controversial issue which raises privacy concerns for many, as MasterCard found to its cost last year when its Advisors Media Solution unit drew a rash of headlines about its plans to sell card transaction data to large retailers.

Recent research across nine countries by privacy group Big Brother Watch found that 41% of people feel consumers are being harmed by big companies gathering large amounts of data, with European consumers far more likely to believe this than those in the rest of the world.

Consumers in South Korea, UK, Australia and France are the most critical of big companies gathering personal data while consumers in Brazil, India and Spain are the most sympathetic to them doing so.

The new Barclays terms also make it possible to tap location data from customer mobile phones to tackle fraud by, for example, tracking their movements if their cards are used abroad.

Another change will see Barclays hold information on customers gathered through contact via social media sites such as Twitter and Facebook.

Read the changes to Barclays T&Cs here: