The ombudsman has told the bank to pay back around £100,000 to almost a dozen customers hit by scams in the first three months of 2019, just before the refunds pledge came in.

It means the victims have had to fight for refunds that the impartial arbiter says should have been awarded straight away, while those customers hit by scams weeks later received payouts almost immediately.

Other customers who were initially denied refunds included a self-employed man who was duped out of £2,000 after a scammer posing as an officer from HM Revenue & Customs called him up and told him he had an outstanding tax bill.

Another customer lost more than £6,000 to an “email interception” scam after he was tricked into paying an invoice from his builder for some home improvement work to a different account operated by fraudsters. One woman lost £2,000 to a fake travel agency when trying to buy return flights to Africa.

In each case the ombudsman told TSB to reimburse the victim’s losses plus interest, concluding that the bank should have asked more questions.

TSB defended its actions, saying it had always made it clear to customers that its fraud refund guarantee would not apply to any losses incurred before April 14 2019.

A spokesman said it was still by far the best bank for reimbursing fraud victims, paying out in 97pc of cases since its refund scheme was introduced and refusing refunds only in instances such as when customers play an active role in scams to abuse the system.

“TSB pays out double the industry average – and recent data shows that TSB has a lower volume of fraud cases with the ombudsman than other banks,” he said.