Lloyds, Halifax and Bank of Scotland customers are unable to use online banking.
Mobile banking through the smartphone apps was also unavailable for most people.
All three banks, which are part of Lloyds Banking Group, tweeted to say they were working to fix the problems and apologised to customers.
The message said: “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.”
The Down Detector website reported that thousands of users were unable to access online and mobile banking from around 4am.