Lloyds and Halifax's systems have crashed, leaving hundreds of people locked out of their accounts.
Website Down Detector has recorded more than 667 issues - with customers unable to see their money or make payments.
On social media, customers are complaining about receiving error messages when they try and log in - reports Mirror Online.
One said: " @AskHalifaxBank is online banking down? Says it can’t show me my accounts right now and all say n/a"
Another said: "Just got through to #Lloyds tech support and they confirmed their whole online system is down. they have no idea when they'll be back up and running."
Martin Lewis explains how you can reclaim thousands from payday loan companies
And a third said: " Is your mobile banking site down. Appalling service post weekend."
Natwest's online banking website also crashed earlier today.
Currently Lloyd's service availability page doesn't list any scheduled issues - although all systems were down over the weekend for maintenance.
It explains that internet banking, app banking and tablet banking were unavailable from 11pm on Saturday April 27 to 5am on Sunday April 28.
Mirror Money contacted Lloyds Banking Group - which owns Halifax and LLoyds - for a status update.
A spokesperson said: "We know some of our customers are having issues with internet banking. We're sorry for this and are working to have it back to normal soon."
