TSB has apologised after customers said they were unable to access its app and online banking services.
Hundreds of customers were impacted by the outage on Monday afternoon which left users unable to see their finances.
The DownDetector platform showed that more than 1,000 customers had reported that TSB's online services were down during the afternoon.
It is understood that banking services in branches and on the telephone are working as normal.
Customers on social media said they were unable to make important payments at the start of the month because of the problems.
A TSB spokesman said: "We're aware some customers are experiencing issues with our mobile banking app and internet banking.
"We're sorry for any inconvenience and are working to resolve this as soon as possible."
Last week, rival HSBC also said sorry to customers who were impacted by an outage to its online services.
The issues for TSB - which has around five million customers - come ahead of the banking firm's £2.65 billion takeover by fellow high street lender Santander.
The move is set to create the UK's third largest bank by the number of personal current accounts.