Barclays is to start selling information about 13 million customers' spending habits to other companies, and has admitted it could share the data with government departments and MPs.

In letters being sent to customers, it is also outlining what details about them it holds and uses which, it said, "may include images of you or recordings of your voice", as well as comments made in interactions with the bank on social media sites such as Twitter and Facebook. Barclays said it may collect "location data derived from any mobile device details you have given us" - suggesting it will be able to pinpoint where in the world a customer is at a particular moment in time.

However, the bank assured customers that any data it passed on to third-party companies would be aggregated to show trends, and that individuals would not be identifiable from it. A spokeswoman said there was "nothing sinister" going on, and added that it would not be profiteering from customers. Like most companies, Barclays has previously used customer data internally, but it has not shared it with third parties before. It is writing to current and savings account customers to let them know about the changes, which will take effect on 9 October.

A leaflet details the "new ways" in which Barclays' companies can use customer data, stating: "We can combine information about you with information about other Barclays customers to create reports which we may share with companies outside Barclays. This information is numerical and not personal, and you will never be identifiable on the basis of it." This could include data on how much people spend on different products and services.

The bank said the data could be passed to government departments and MPs – for example, to give them an insight into what was happening in their constituency.

In a statement the bank said: "We only use information in a numerical, anonymised and aggregated way, as is standard practice at many companies. It is not about providing information for sales or marketing use and does not include any personal data."

It said the move was in accordance with industry guidance from the Information Commissioner's Office and the law. "Customers are always able to opt out of marketing activity and their personal data will never be passed on to anybody else without their explicit consent," it added.

The bank said that data relating to where a mobile phone was at a particular time would be used for fraud prevention purposes, and only when a transaction was picked up by its fraud detection systems. It would confirm "at a country level" if the customer was in the region where the suspicious transaction had taken place. Customers will be able to opt out of this if they wish.

Barclays is the latest in a line of companies to come under scrutiny over the way they use customer information. It emerged recently that Tesco is using data about what Clubcard holders buy in its stores to serve targeted ads to online users of its new movie streaming site, Clubcard TV. Tesco also plans to use its Clubcard data to tackle obesity by offering customers "tailored suggestions for how they could shop more healthily".

Financial companies have different policies when it comes to the use of people's data. For example, MasterCard states in its global privacy policy that it will "perform data analyses" and offers the chance to opt out on its website. Otherwise, people are automatically opted in.