Barclays' mobile and online banking services went down for around two hours on Tuesday.
The giant UK bank confirmed the problem on social media, informing its millions of customers that they could still use their cards and cash machines as normal.
The down detector website recorded thousands of user reports during the morning, with two thirds relating to the mobile banking app.
In a tweet at around 1pm Barclays said that it had fixed "some technical issues," adding "we're so sorry it took us so long to fix the issue".
While the bank faced grumbling from frustrated customers, it also received several positive tweets.
Wrote @rsrob1990: "thanks i know a lot of people will be angry over the outage, but i know you guys were doing your best to resolve it."