The bank said that the issue had been resolved but a number of customers took to Twitter to flag that they were still unable to access their accounts. Some said computers were also down in their local branches.

"Why are you still insisting that the app is fixed?" One customer, Yvonne Nicol, wrote on Twitter to TSB. "Listen to your customers..."

Others threatened to close their accounts with the bank following the outage. It occurred after TSB, owned by Spain's Sabadell, updated its mobile app, but a spokeswoman told Reuters the issues were unrelated to this.

The bank had apologised for the outage in a statement sent via email and to customers on Twitter and said the problem had been fixed, with services working as normal.

"We're seeing customers logging into our online banking in line with normal levels," a spokeswoman told Reuters in response to the continued customer complaints.

The latest outage comes just as TSB was returning to normality after an attempt to switch its customer base over to a new IT system left thousands of customers locked out of their accounts, with some unable to access their accounts for over a week or make vital payments, and others falling victim to fraud.

The crisis and the bank's handling of it have tarnished the reputation of the mid-sized lender, which was just gearing up to try to win more market share in a sector dominated by the four biggest lenders.

In June, a powerful committee of lawmakers called on the TSB board to consider whether CEO Paul Pester should stay in his job, saying the bank's public communications following the outage had been complacent and misleading.

Pester and TSB Chairman Richard Meddings were called before the committee twice, with lawmakers raising concerns the bank had downplayed the scale of the problem.

An early report into what went wrong by International Business Machines Corp, called in by TSB to help fix the issue, concluded that inadequate testing of the new system had contributed to the outage.

The bank has also commissioned law firm Slaughter & May to produce a report into the outage, which will be published with commercially sensitive information redacted once it is complete.

(Reporting by Emma Rumney; Editing by Keith Weir)

By Emma Rumney