Lloyds, Halifax and Bank of Scotland customers were left without mobile and online banking for hours on New Year's Day.
Banks in the Lloyds group suffered an outage on the app and website which took several hours to fix.
Some customers voiced their frustration on Twitter with one saying: "Banking app is down! Just when I was feeling brave enough to check my balance after Xmas!"
In a statement issued in the early afternoon, Lloyds Bank apologised and said internet and mobile banking had gone back to normal.
Account holders were able to use cash points and telephone banking services during the online outage, but no branches were open because of the bank holiday.
Customer reaction was mixed, with one person joking: "Obviously the millennium bug overslept by 20 years."
While many were frustrated and suggested the problem is a frequent occurrence, some also shared appreciation for the engineers working to fix the issue.
It comes after TSB customers suffered weeks of problems in 2018 owing to a long running IT problem.
Banks have been warned by the regulator that outages should be fixed swiftly.