Barclays, which has some 24million customers, has yet to give specific details on what caused the issue or when the service will resume normally.
The bank's website was not allowing customers to log in.
Meanwhile customers of online challenger bank Cashplus - which targets people with poor credit histories - were unable to access their accounts, make cash withdrawals, or make or receive payments earlier this week.
Angry customers were quick to complain. "Steven Hyde tweeted: "@barclays I can not access online banking with your current problems and need to pay a bill. "Can I use my my debit card to pay or is this also affected?"
Some customers tweeted their frustration throughout the day. "Our Barclaycard Telephone Banking team are working as they should be". How do you intend to compensate yr customers? "The only bank where it happens".
The mobile app is working as normal, the bank said.
Online and telephone banking were hit by the problems, which started on Thursday morning. "Sorry and thanks for your patience".
She said: "While Cashplus may not be regulated as a bank, it claims to offer "the UK's most seamless banking services experience".
The spokesperson added: "Everything is now back up and running and we're really grateful for customers bearing with us".
"We're now experiencing some technical problems - customers are still able to use Barclays Mobile Banking app to check their balance, make and receive payments", the company added.