A pensioner lost her £90,000 life savings when fraudsters claiming to be Scotland Yard police convinced her they wanted to protect her bank accounts.
The case is highlighted in today's first episode of the new series of Rip Off Britain on BBC One.
North East woman Joyce Smith, 84, said the fraudsters constantly phoned her, giving her strict instructions on how to avoid the loss of her savings, but instead convinced her to make three separate transactions to them.
"I thought I was talking to the police, and I've always considered the police trustworthy," Ms Smith said.
Despite normally hanging up on cold callers, Ms Smith took the advice of the fraudsters, who said she should be "wary of any questions from the staff at her local branch" because they were supposedly in on the con.
The caller, who called himself Sergeant Jackson, told Ms Smith he had set up safe accounts in her name, but she would need to visit her local branches to transfer the money to them.
Ms Smith followed their instructions, leading her to transfer almost £32,000 from her accounts at two separate banks.
Although her banks noticed the unusual transactions, they could not notify her and stop them going through because she went on a long-planned holiday.
Upon her return, the fraudsters contacted Ms Smith, again, convincing her to transfer another £58,000.
Unsure about the validity of their story, she contacted the real Scotland Yard, but she was too late - her life savings were gone.
"I've learnt to trust nobody about anything. That's a terrible thing because I always trusted everybody," she tells the programme in an attempt to warn others.
The police were able to recover £13,000 of her lost savings, but because she transferred the money on her own accord, the banks were not obliged to help her recover the rest.
Gloria Hunniford, the presenter of Rip Off Britain alongside Angela Rippon and Julia Somerville, said she herself was a recent victim of bank fraud.
"This is a particularly upsetting story, and we're very grateful to Joyce for sharing it with us so nobody else has to go through the same thing."
Police have stressed they will never contact people asking them to transfer money from one account to another, and that anyone who receives such a call should report it to the police and their bank straight away.
* The new series of Rip Off Britain is on BBC One at 9.15am today.