The consumer champion said that some banks regularly blame customers for missing generic warnings or not doing enough to realise they are being scammed.

Bank transfer fraud, where consumers are tricked into sending money to criminals, resulted in £317m in losses last year.

Most of the major banks and building societies signed up to a voluntary code of conduct which requires them to reimburse victims who were not at fault.

But Which? said refunds were paid in just 41 per cent of cases and that victims are being treated unfairly, with banks having “unreasonable expectations” of what they should have done to protect themselves.

UK Finance, the banking trade body, said the code has more than doubled the amount being repaid to victims and that anyone who was not satisfied with an outcome can complain to the Financial Ombudsman Service.

But Gareth Shaw, from Which?, said: “Our analysis has found clear issues with how banks are meeting [the code’s] core objective of reimbursing blameless people who have lost money through bank transfer scams.

“Even as this type of crime continues to surge, the lack of fairness, consistency or transparency across the industry means that the chances of people getting their money back is often a total lottery.”

He said the code should be made mandatory to better protect victims.

Which? said banks are relying on general warnings shown before a transfer is processed but that these needed to be more “rigorous” and that victims can easily be manipulated to ignore them by sophisticated fraudsters.

Katy Worobec, managing director of economic crime at UK Finance, said consideration should be given to how data breaches in other industries facilitate fraud and said there needs to be an overall strategy.

She added: “The banking and payments industry continues to take action on all fronts to stop these crimes from occurring in the first place, working with law enforcement and the government to protect customers and prevent stolen money from going to criminals.”