About 700,000 Nationwide customers were informed that debit card transactions made on Tuesday withdrew funds from their accounts twice -- on Tuesday and again on Wednesday.
This put many accounts into arrears, The Daily Telegraph reported Friday.
At Natwest, 7.5 million customers were caught in computer glitches that prevented them from using debit cards or accessing their accounts online.
"Nationwide must commit now to compensating those who have incurred fees or other costs including phone calls to put things right," said Mike O'Connor, chief executive officer at Consumer Focus.
"Natwest online banking down again. Really? Really? Can't pay my bills. Can't take out money," wrote one bank customer, Owen Thomas, in an online posting.
Nationwide said no customer would suffer financial losses from the problem, which it said was caused by "human error." Embarrassing for the bank, it recently went on an advertising binge to take advantage of a publicized computer glitch at a rival bank.
Natwest said in a statement, "Some of our customers are experiencing technical difficulties with online banking and debit cards. We are working hard to resolve this and will provide an update as soon as possible."
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