The customer complaints body is following the lead of the Financial Conduct Authority in updating its understanding of of APP fraud, which happens when businesses or individuals are conned into sending money to a fraudulent account to pay for goods or services.

The FCA is currently consulting on requirements for firms to handle APP complaints in line with complaints handling rules in the FCA Handbook and to provide the victims with access to the Ombudsman.

UK Finance data on APP fraud show there were 43,875 cases of APP fraud and total losses of £236 million in 2017, only a quarter of which was refunded by financial services firms.

Caroline Wayman, chief Ombudsman, says that in the future it will expect to see "clear evidence" that banks have investigated thoroughly - and reflected hard on what more might have been done to protect their customers and their money.

"We often hear from banks that their customers have acted with 'gross negligence' - and this means they’re not liable for the money their customer has lost," she says. "However, gross negligence is more than just being careless or negligent. And as our case studies show, the evolution of criminals’ methods - in particular, their sophisticated use of technology and manipulative “social engineering” - means it’s an increasingly difficult case to make."

The Ombudsman's harder line comes as the UK's Payment Systems regulator works with banks and consumer groups to develop an industry code for reimbursing victims of APP scams.