Nationwide, HSBC, Barclays and Virgin Money customers have been hit by payment problems, leaving many unable to send and receive money.
This is not the first time a major UK bank has faced technical issues - last month, Natwest's app saw an outage which affected 10 million customers.
The issues have been compounded as Friday is the last working day of the month and when many are paid by their employer. Many also have direct debits leaving their accounts to pay bills.
Subscribe to MoneyWeek
Subscribe to MoneyWeek today and get your first six magazine issues absolutely FREE
Sign up to Money Morning
Don't miss the latest investment and personal finances news, market analysis, plus money-saving tips with our free twice-daily newsletter
Don't miss the latest investment and personal finances news, market analysis, plus money-saving tips with our free twice-daily newsletter
HSBC UK said on its website: "We're really sorry that some customers are having issues accessing personal online and mobile banking. Our IT teams are working hard to get these services back to normal.”
Virgin Money said access to its app has been restored after issues in the early hours of Friday but that there is a backlog of payments to process.
Meanwhile, Nationwide told the BBC it was aware of "third-party payments issues impacting some providers, including Nationwide" which had "delayed a small number of payments".
Nationwide added that it expected these to be "processed and paid later today.”
Virgin Money said on X, formerly Twitter, that "like other banks" it is "working hard to process the backlog of payments delayed as quickly as possible".
Banking glitches - how long will they last?
Customers have taken to social media to vent about the problems. One customer said on X: “Very embarrassing 20 minutes this morning when my card was declined for a trolley load of shopping because the money I transferred earlier hadn’t arrived into my account. No cash machines either.”
It is understood that Nationwide, HSBC, Barclays and Virgin Money were affected by issues with the Faster Payments service, which allows UK sterling payments to be made electronically between banks on the same day.
It is understood the issue has now been fixed, although a backlog of payments is still causing issues.
Pay UK, which oversees the system, said: "We are aware that a small number of Faster Payments were delayed or not processed earlier today.
"Working with our infrastructure provider, this technical issue has been fixed."
This is not the first time the banks have experienced problems. In March, Nationwide customers reported a range of issues, including not being able to transfer money, while in November HSBC was forced to apologise after thousands of its UK customers were unable to access mobile and online banking for more than 24 hours.
What can you do if your bank has an IT glitch?
If you have made a loss because of the banking glitch, you should contact your bank and ask them to put it right and to cover any losses you may have made.
For example, you may have got charged for a late payment, or paid to travel somewhere but were unable to pay for your goods or service.
According the the Financial Ombudsman, the bank must respond within 15 days and if you are unhappy with the response, you can take the matter to the ombudsman for further investigation.
