Customers of Lloyds, Halifax and Bank of Scotland have been unable to access their accounts after the group's online banking services crashed.
Websites and mobile apps for the three companies, part of Lloyds Banking group, were all unavailable.
The banks apologised and said the issue was to do with the log-in process and telephone banking could still be used.
Customers began reporting issues at around 04:00 GMT but was resolved by about midday.
@LloydsBank any idea when internet banking will be accessible again??I need to transfer some money over for tomorrow!— Amy Nykyforczuk (@Lyubacn) January 1, 2020
@HalifaxBank why is the mobile banking app not working?!?! How long until it is back up working again?— Ainsley (@XtremeAinz) January 1, 2020
The Lloyds Group has more than 13 million active online customers - and more than nine million are active mobile users.
The three banks all posted the same message on Twitter: "We know our customers are having issues with internet and mobile banking. We're sorry about this and we're working to have it back to normal soon."
In response to one customer who asked whether the bank will be responsible for any late fees that they might get as a result of the problem, Lloyds Bank tweeted: "If you do receive any late payment fees as a result please let us know and we will look into for you at the time".
In October, MPs condemned the level of IT failures at banks, saying in a Treasury Committee report that the frequency of online banking crashes and customer disruption was "unacceptable".
Major banks typically suffer more than 10 online outages a month.