Scams involving bank transfers have seen one individual lose almost £400,000 in a single incident, according to a consumer group which wants to see banks shoulder more responsibility for such frauds.
Which? has been gathering evidence for a "super-complaint" it made in September to regulators about bank transfer fraud.
It wants to see banks take more responsibility when people are tricked into making bank transfers to fraudsters.
Across the UK, consumers now make over 70 million bank transfers a month.
But Which? said victims conned into transferring money by bank transfer to a scammer currently have no legal right to get their money back from their bank, unlike with payment methods such as credit and debit cards.
In just two weeks since Which? launched an online scams reporting tool, it said more than 650 people had told it about collectively losing over £5.5 million to bank transfer scams.
The largest single loss reported to Which? was £395,802, with most people losing £1,200 on average.
Which? said it has seen evidence of "incredibly sophisticated scams, where consumers could not be expected to spot they were the target of fraudulent activity".
Alex Neill, managing director of Which? Legal Services, said: "We're only seeing the tip of the iceberg, so it is vital that these stories are heard and action is taken to tackle bank transfer fraud.
"The harsh reality is if you're conned into making a payment to a fraudster via bank transfer you often stand to lose large sums of money.
"We expect the financial regulators to use this opportunity to take meaningful action and propose new measures to help the victims of bank transfer fraud."
On September 23, Which? used its legal powers to make a super-complaint to the Payment Systems Regulator and alert the Financial Conduct Authority.
Which? wants regulators to formally investigate the scale of bank transfer fraud and how much it is costing consumers, and take action to propose new measures, and greater liability for banks to ensure consumers are better protected when they have been tricked into making a bank transfer.
People who have been the victim of a bank transfer scam can use the Which? tool to share their experience with the regulator www.which.co.uk/bank-transfer.