Santander UK customers were unable to access their online accounts for several hours from Friday morning until mid afternoon.
“We are currently having problems with customers logging into Online Banking,” the lender told customers via Twitter.
It comes before a holiday weekend and monthly salary payments for millions of Brits.
The bank said the problem had been solved.
“We apologize for the problems some customers faced today using our digital banking services,” the lender said in a statement. “These have now been resolved and we can confirm that all payments due today have been processed.”
He said debit cards, credit cards, and ATMs should all work during the outage.
While disruptions lasting less than a day will be a minor nuisance for most customers, there may be significant issues for those planning large transfers before large purchases, such as for a car or home.
Sophie Rivett, of Worcester, told the Palestinian Authority news agency she had trouble paying bills as she moved into a new home on Friday.
He said, “I’m in the middle of a move, I had to borrow money from the family.”
Some frustrated customers have left comments on the Downdetector website, which monitors IT issues across different services.
“Phew. I just walked in and paid two bills. Oh, the relief,” said one user after it was reported that the problem was fixed.
“But really – not good enough, Santander … you don’t move on with things at the end of the month. How about a fee for your frustrated customers?”