An elderly woman was conned out of £19,000 over three days by a fraudster posing as a police officer.
The victim was called by a man claiming to be a Met detective, who said her bank card had been 'compromised'.
He asked her to hang up, dial 999 and ask to be put back through to him, but he did not hang up so she remained on the same line.
He then told her he was also investigating counterfeit currency at her bank and needed her help to uncover evidence.
He persuaded her to withdraw thousands of pounds from her savings account over three days, between Monday (January 6) and Wednesday last week, and hand it to a courier at her doorstep.
Police this week warned people to beware of such scams, which they said were becoming prevalent in the area.
A Hounslow Police spokesman said: "Whilst we are doing everything we can to trace and arrest these suspects the key message to the public is that neither the police nor the banks will ever ask you to withdraw money or to hand over cash or bank cards or ask for any confidential financial information.
"If you do receive such a call contact the police as soon as possible on either a separate phone line or if using the same phone line wait at least five minutes so that you are no longer connected to the suspect."