HSBC has apologised to customers after it suffered online banking glitches for two days running.
Personal customers struggled to log in, while business customers had problems online as well as with the bank's mobile app.
HSBC said the problems were due to a "complex technical issue" with its internet banking systems. It said that any fees customers rack up as a result of the problems will be waived.
The bank said the hitches were not caused by a cyber attack or any other malicious act.
John Hackett, HSBC's UK chief operating officer, said: "I'm pleased to say that we have seen a steady return of service to internet banking in the past few hours for our personal and business customers.
"We will be monitoring the service very closely, ready to respond should any new issues arise.
"It is encouraging that more and more customers have been able to log on this afternoon, and all customers should feel free to try.
"I'd like to apologise once more for any inconvenience that this technical issue has caused. We will be waiving any fees incurred as a result of this incident, and we will be happy to help all our customers with any issues that have arisen.
"Thank you for your patience."
Earlier, Mr Hackett said: "We profoundly apologise for any inconvenience this has caused."
HSBC has 17 million personal and business banking customers across the UK. It is not known how many of them have been affected by the problems.
Andrew Tyrie, chairman of the Treasury Committee, said he will be asking for an explanation.
He said: "The frequency of these failures across the financial services sector suggests a systemic weakness in IT infrastructure. This is concerning.
"I will be asking the chief executive of HSBC, and the regulators, for an explanation of these failures, and action taken to sort them out. They just keep coming."
HSBC said earlier in a message on Twitter: "Good morning, unfortunately we are experiencing further service issues with personal online banking, however our Mobile App is available."
On Monday, HSBC said business and retail customers were unable to access online and mobile banking, due to problems which started at 8am.
It later posted a message on Twitter to say that its online service was back up and running - to which one Twitter user replied: "Oh no it isn't!"
Other Twitter users vented their frustration, with one saying: "Still unable to access online banking today, not great when you need to check bills have been paid after a long weekend!"
Another Twitter user wrote: "Seriously. It's beyond pathetic."