TSB could be in line for a hefty fine after a botched system upgrade left customers unable to use online and mobile banking services for a fourth day and sparked fears of a data breach.

Customers of the challenger bank took to social media to vent their anger after outages prevented some from making transfers, paying bills and checking balances.

Others claimed that when they did manage to log in they were able to access other people’s money.

Niall McDonnell, 28, told The Daily Telegraph that he was able to see a business account on his profile when he logged in, which contained over £2.8m. He sent a screenshot to the City watchdog and data regulator the Information Commissioner's Office (ICO).

Another customer said she was able to see the account details of four other people. Ashley Tait, 25, said that her father was able to see her account details and balances when he logged in to his own account.

TSB customer Matthew Neal told the BBC he was given access to someone else’s £35,000 savings account after logging on to his account on Sunday evening to check how much he'd spent the night before in the pub.