TSB customers complained on Wednesday that they were unable to access their online banking services
Several TSB customers were unable to access online banking and mobile applications for Android and iOS on Wednesday.
Independent website DownDetector, which tracks social media posts on the performance of the sites, showed hundreds of customers complaining about an outage.
Users were met with an "unexpected error" and some said they had no access to their accounts.
TSB said it was "working to resolve" the problem as soon as possible.
Many of the customers said that the Covid-19 outbreak left them more dependent than usual on being able to use their banking applications and online services.
One user tweeted: "It has been inoperable since 6:00 am, unexpected errors arise. Joining a new bank. It seems like a weekly thing that the TSB is inoperative."
Other added: "This is not the time for your application to be inactive @TSB".
A TSB spokesman told BBC News that the bank hoped to resolve the problem "as soon as possible".
"We are aware that several customers are currently experiencing problems with our mobile application and internet banking," he said.
"We are working to resolve this as soon as possible and apologize for any inconvenience caused."
It is not clear how many customers were affected by the problem.
Those affected were advised to try to leave and enter again.
The last outage came after an IT failure in November, which meant that wages and other payments were not paid on the accounts of some TSB customers.
The bank said the incident occurred due to a "processing error".
In April 2018, a similar IT failure left up to 1.9 million TSB customers unable to bank online for several weeks.
Customers were moved to a new system, but an investigation found that it had not been properly tested before going live.
It cost TSB a total of £ 330 million in customer compensation, fraud losses and other expenses.