Santander UK customers were unable to access their online accounts for several hours from Friday morning until mid-afternoon.
"We are currently experiencing issues with customers logging in to Online Banking," the lender told customers via Twitter.
It comes ahead of a bank holiday weekend and monthly salary payments for millions of Britons.
The bank said the problem has now been fixed.
"We're sorry for the difficulties customers experienced earlier today accessing our digital banking services," the lender said in a statement. "The issues have now been fixed and our services are now working correctly."
It said debit cards, credit cards and cash machines should all have been working during the outage.
While outages that last less than a day will be a minor nuisance to most customers, there could be significant knock-on problems for those planning on large transfers ahead of big purchases, such as for a car or home.
Sophie Rivett, from Worcester, told the PA news agency she had problems paying her bills as she moved into a new house on Friday.
She said: "I am in the middle of moving house, had to borrow money from family."