Just under two million TSB customers were unable to carry out online banking transactions on Wednesday morning after maintenance work overran.
It is at least the third time this year that account-holders have been hit by technical problems.
Maintenance work was due to finish at 06:00 GMT on Wednesday morning, but online banking was not restored until after 10:00 GMT.
However customers were still able to bank via the mobile app, the bank said.
TSB said, "Online banking is now available and customers have been able to use the mobile app as normal this morning. We're really sorry the planned maintenance overnight for our online banking took a little longer than expected, and apologise for any inconvenience this may have caused."
It follows an IT fiasco in April, when 1.9 million customers were unable to use their accounts for several weeks.
TSB reported further issues in September, again after maintenance work was carried out.
Paul Pester, TSB's former chief executive, was forced to step down because of the systems failure, and after he was accused of misleading MPs on the Treasury Committee.
TSB lost a net 16,000 customers in the three months following the IT debacle, out of a total of around five million.
The Financial Conduct Authority is still investigating the problems that occurred in April, and will consider fining the bank as a result.