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One of the UK's biggest banks has said 70% of its customers who fall victim to a scam do not get a single penny back.

Figures from the RBS Group seen by BBC Radio 5 live show the extent to which victims are losing out to scammers.

From January to September this year almost 5,000 of the bank's customers fell victim to various scams - at a total cost of more than £25m.

The bank says the average cost of falling for a scam has gone up by 40% since 2014, to more than £13,000.

Huge crime wave

The rate at which people are suffering frauds like this is increasing according to the bank, with 900 cases reported in the third week of October alone - compared with 739 for the whole of September.

Conservative MP Mark Garnier, a member of the Commons Treasury Select Committee, said: "It is interesting that overall crime is going down, but we still clearly don't know what is happening with crime online."

Customers of Natwest, part of the RBS Group, who have been scammed are most frequently caught out by "vishing" - verbal phishing - where they are tricked into giving their account details during a telephone call.

The rules, according to the Financial Ombudsman Service

"The problem with complaints where consumers have been conned in to making the transactions on the fraudster's behalf is the bank is not generally responsible for the fraud, unless their advice, delays or other errors have resulted in the money being stolen.

"Some banks may make a pragmatic decision to refund some of their customers where they have clearly been defrauded. However, when this happens, it is important to bear in mind that they are under no obligation to do so.

"Where consumers have not authorised transfers - for example where their card has been stolen or they have been tricked in to handing over passwords or codes allowing the fraudster to make the transactions themselves, the regulations say that the bank can only debit the consumers account if they have been 'grossly negligent'."

Some customers are also increasingly falling victim to so-called romance scams, where the victim is befriended online and then conned into handing over money.

Mr Garnier added: "There is obviously a huge crime wave that isn't being reported, either people aren't aware, or are embarrassed to report it."

Image copyright Getty Images Image caption Some customers may be embarrassed to report being defrauded

Growing threat

Terry Lawson, head of fraud at RBS, told BBC Radio 5 live: "These figures show that the threat of scams is growing.

"The means by which fraudsters trick individuals into parting with their money is becoming more sophisticated, but it always ultimately depends on the individual transferring money out of their account.

"We would never contact a customer asking them to transfer money. If a customer of any bank is contacted by someone asking them to do this they should simply end the conversation and report the matter to the police."

Customers' bank account and sort code details were put at risk by the recent alleged cyber attack on TalkTalk.

The telecoms company said the attack was "smaller" than originally thought, but that bank details may have been accessed.

An inquiry into the hack will be launched by Jesse Norman, chair of the Culture, Media and Sport Select Committee.