Some TSB customers were unable to access online banking on Monday.
Users on both the app and the website received error messages while trying to log into their accounts.
There are dozens of social media comments stating that affected customers have not been able to get past the security questions, some for several hours.
The bank said it was working to fix the problem as quickly as possible and that only a "very small number" of people were affected.
Several users reported that their accounts had been blocked while trying to log in with the correct details.
Others have complained that they cannot access support for this issue over the phone or via Twitter.
It was also suggested that the difficulties had started over the weekend.
A TSB spokesperson told the Moneysaving Expert website:
"We are aware that a very small number of customers have encountered problems accessing their Internet banking today.
"We are working to resolve this issue as quickly as possible and apologize for any inconvenience this may have caused. Our mobile banking app remains fully functional."
It's unclear how many customers were affected by this issue, but it comes just months after an April outage affected hundreds of users.
And last year, a "processing error" meant wages and other payments were not being paid into the accounts of some TSB clients.
In April 2018, a similar IT error prevented 1.9 million TSB customers from doing online banking for several weeks.
Customers were moved to a new system, but an investigation found that it had not been properly tested before it was released. It cost TSB a total of £ 330 million in customer compensation, fraud losses and other expenses.
