An urgent protection warning has been issued to customers of HSBC, Lloyds Bank, Santander, TSB, Nationwide Building Society and Virgin Money.
It comes as a Which? investigation suggests the banks may be leaving customers vulnerable to fraudulent spoofing attempts. This is when a scammer attempts to impersonate legitimate companies - such as banks - to deceive victims. Scammers often forge the name and/or number that shows on an email or call from the legitimate site or retailer.
However, in a bid to made it more difficult for fraudster to impersonate them, companies can sig up to Ofcom's Do Not Originate (DNO) list. The regulator then shares this list with telecoms providers to help block potential scam calls and messages.
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In a bid to test the effectiveness of the protection afforded to customers by leading banks, Which? made calls to a test phone, spoofing the prominent numbers of 14 account holders, reports Liverpool Echo.
The consumer group chose numbers that were printed on the back of debit cards or listed as fraud helplines on bank websites, calling at least one number from HSBC, Lloyds Bank, Santander, TSB, Nationwide Building Society and Virgin Money. And all were successfully spoofed.
Previously, Which? found that two thirds (68 per cent) of victims said the incident involved number spoofing.
Ofcom recently strengthened rules and guidance for telephone networks involved in transmitting calls - from both mobiles and landlines - in a bid to identify and block spoofed calls and make it more difficult to scammer to use the spoofed numbers.
Rocio Concha, Which? director of policy and advocacy, said: "Number spoofing is a particularly malicious form of fraud used by scammers to deceive their victims - and our research shows some banks could potentially be leaving their customers at risk."
A spokesperson for trade association UK Finance said: "Protecting customers from fraud is a top priority for the finance industry which is why we are actively working with the regulator Ofcom to help crack down on number spoofing.
"This initiative prevents criminals impersonating banks by protecting bank inbound phone numbers from being used to make outbound calls and socially engineer or scam bank customers. It is important to remember that anyone can be caught out by these criminals and that you should always stay alert.
"To help stay safe, customers should always follow the advice of our Take Five to Stop Fraud campaign and question any uninvited call requesting their personal information or money in case it's a scam." A HSBC spokesperson told Which?: "We are participants of the Do Not Originate scheme which provides additional protection, alongside numerous other measures, to help protect customers from scams and fraud.
"We regularly review the numbers we have registered with a view to additional entries where it is appropriate to do so. We are currently in the process of adding those two numbers to those already on the register."
Lloyds Bank said: "Banks can't solve the problem of number spoofing alone and telecoms firms need to speedily address the technical gaps in their systems that allow this type of fraud to happen, even with Do Not Originate lists in place."
A Nationwide spokesperson told the consumer group: "Nationwide takes the protection of its members seriously and our contact numbers are on the Do Not Originate list - and therefore cannot be spoofed. However, it appears one of our numbers was inadvertently missed, for which we would like to thank Which? for bringing to our attention. We can confirm this is now being added to our list of protected numbers for future."
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A Santander spokesperson told Which?: "Thank you for bringing this to our attention. We have now requested that Ofcom adds this number to the DNO list.
"As part of the measures we take to protect customers against fraud, we aim to include all our inbound-only customer service phone numbers on the DNO list, which provides some protection against spoofing but is not 100% comprehensive."
TSB said that all relevant TSB numbers are now on Do Not Originate.
A Virgin Money spokesperson said: "Virgin Money currently has over 40 numbers registered for the Do Not Originate service and we continue to add numbers to this to ensure as much coverage as possible.
"The list is not a guarantee that spoofing won't occur as not all providers use the list and technology constraints can mean that some calls get through, however, we will raise this with them and ensure that all the numbers you highlighted are registered."
Rocio Concha, Which? director of policy and advocacy, said: "Number spoofing is a particularly malicious form of fraud used by scammers to deceive their victims - and our research shows some banks could potentially be leaving their customers at risk."
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