A man aged in his 70s was left "devastated" when he was conned out of nearly £300,000 by phone scammers.
He unwittingly enabled a bogus caller to transfer the cash into fraudulent accounts, say North Wales Police.
The caller had told him the two banks he used were involved in a fraud case and it would be safer to transfer his savings into another account.
Police are using the victim's case to highlight financial fraud amid warnings for people to be on their guard.
PC David Hall, financial abuse safeguarding officer, said the force was dealing with "several calls every day", with fraudsters targeting people via email and phone calls as well by letter and in person.
He said the amounts involved varied with the highlighted case one of the worst, coming at the end of last year, when the man from north east Wales, who does not want to be identified, unwittingly handing over £270,000.
The force said a fraudulent £25,000 loan application was also made using his details.
PC Hall said such scams take place across several calls over days or weeks, with criminals tricking people into passing their banks' scrutiny checks by giving them the prompts to answer their questions before their money was transferred.
'Fraudsters very plausible'
"As you can imagine, this victim was absolutely devastated when the horrible truth dawned," he said.
"The scammer promised the money would be returned to his account once the spoof investigation was completed but, of course, it never was.
"At no point during those couple of weeks did the victim suspect anything was wrong."
The officer said frequent cases involve phone calls where scammers claim to be from the police or a bank's fraud department and ask them to move money to "safe accounts", pretending people working in their bank are involved in fraud.
North Wales Police and Crime Commissioner Arfon Jones branded the perpetrators the "lowest of the low", adding that while victims were not "particularly gullible", the fraudsters were "very plausible".
He advised anyone who received a suspicious call or email to contact Action Fraud, the UK's national fraud and cyber crime reporting centre.