John Thompson, 91, lost his life savings after he was tricked into transferring three payments of £4,900 out of an account he had for more than 50 years.
The former police officer, who lives in Sprotbrough, Doncaster, was contacted by a man claiming to be from the security department of his bank.
Mr Thompson uses free call blocking software on his landline which can be sued to screen out unwanted telephone calls, but the scammers used sophisticated methods to make it look like they were calling from his bank's local branch number.
"First of all I put the phone down because I thought it could be a scam," Mr Thompson said.
"Then about 20 minutes later the phone rang again and the chap said he was the bank manager at the Doncaster branch."
On this call, the "bank manager" told Mr Thompson there had been a security breach on his account and he needed to transfer his funds into a new, safer account. The man was able to give him personal information including his account balance, most recent payments and even his late mother's maiden name.
"I realised, after I sat down and thought about it, that it could be a scam," said Mr Thompson, who was an assistant inspector with Kenya Police in the 1950s.
"I called in to my local branch the following day and sure enough, they informed me it was a scam."
Unfortunately, despite a number of meetings, letters and phone calls, Mr Thompson has been unable to get a refund for his loss from the bank as it is seen as an 'Authorised Push Payment', meaning he had moved the money himself rather than it being stolen out of his account.
He said: "I do feel rather foolish to think I did this and it was a scam - but they were so convincing.
Mr Thompson has since received support from Action Fraud and South Yorkshire Police's Fraud Protect Officer Andy Foster, who spends his time raising awareness of fraud and scams across South Yorkshire and visiting vulnerable victims to offer them support and fraud prevention advice.
PC Foster said: "Sadly, Mr Thompson's story is all too familiar and I have met countless victims of similar telephone scams.
"These fraudsters are professionals, using increasingly sophisticated methods to trick people out of their hard-earned money and life savings. The fact they are now able to make it look like they are calling from a local number, or the number of your bank, makes their devious schemes even more convincing.
"A legitimate bank would never contact you to ask you to transfer your money into another account. If you are in doubt, hang up the phone, leave it for a few minutes and then call your bank for advice."