The Financial Ombudsman Service (FOS) said banks regularly used the excuse that customers were “grossly negligent” as a reason not to refund their money.

But the ombudsman service said there was “a very high bar” for being grossly negligent because scams are becoming more sophisticated and difficult to spot.

Caroline Wayman, chief executive of the Financial Ombudsman Service, said: “Each year we see more than 8,000 cases involving fraud and scams – everything from disputed cash withdrawals and identity theft, through to mobile phone SIM-swaps and fake banking websites. And where criminals are involved, both banks and their customers often tell us in strong terms that they haven’t done anything wrong.

“But it’s not fair to automatically call a customer grossly negligent simply because they’ve fallen for a scam. That’s especially true in light of the sophisticated way criminals exploit banks’ security systems – and convince customers that their money is at risk.”

In one example, a customer called Brian contacted the FOS after his bank refused to refund him money that was stolen from his account in a text message scam.

The bank said because he’d given out his security details he had been grossly negligent, so it wouldn’t refund him the £7,000 he’d lost.

But the FOS reviewed his case and said it was a sophisticated fraud so told the bank to reimburse him the full amount.