TSB has apologised to customers who could not access their accounts through the company’s app and online banking service on Sunday night and Monday morning.

A number of customers complained of a “data breach” and said that they were able to view other people’s account information through the app.

The problems came after TSB carried out planned upgrade work to its technology over the weekend. The bank had warned that some services would not be available from 4pm on Friday to 6pm on Sunday but customers found that problems persisted beyond this, leaving them locked out of their accounts.

One customer told the BBC that he had been credited with a large sum of money that was not his once he managed to get back into the app.

“My balance, because of my overdraft, is in minus, but my balance was showing at £13,000,” said Laim McKenzie, from Paisley in Scotland.

In a statement on Monday morning, TSB said that there were still “intermittent issues” with its mobile and online banking services and that it was “working as hard and as fast as we can to get these up and running”.

The announcement came after TSB’s social media team faced a series of complaints from customers on Sunday evening.

Craig Malcom tweeted: “TSB I currently have access to £20k+ of other people’s money.

“I suggest somebody answers the phones as i've been on hold for 45 minutes!

“This is a MASSIVE breach of data protection! If i have access to their account they could have to mine as well!”

Another Twitter user called Bex wrote: “@TSB so go to my app and have someone else’s accounts there!!!! Serious DPA (Data Protection Act) breach!”

“Want to speak to someone now and a half an hour wait!! What would the FCA [Financial Conduct Authority] say about this!!! Might just inform them a bank is giving away other people’s account numbers.”