Police are warning people of a new vehicle theft scam in Hertsmere after a car was stolen.
Criminals have been answering car adverts, and when they view the car they ask to see the spare key and swap it for a fake one that looks genuine.
They then later return and steal the car.
This month a grey Audi estate and a silver Vauxhall Insignia have been stolen in Bushey and Borehamwood by offenders using the method.
On Monday, March 17 two men went to see the Audi in Hillside Road in Bushey. The car was then stolen between 11pm on Tuesday March 18 and 7am on Wednesday 19.
One of the men who went to see the car is described as white, 6ft 1in tall, of medium build, with short brown hair.
He had a Polish accent and was wearing black jeans and a jacket. The second man is described as Asian, 5ft 9ins tall, of medium build with black hair.
Detective Constable Rob Fleming from Hertsmere Local Crime Unit said: "We want to make people selling their cars aware of this scam.
"If your prospective buyer wants to see your spare key, refuse and please contact police straight away on 999.
"If you recognise the descriptions of these offenders or know who they may be, we would like to hear from you. I would also like to hear from anyone who has been selling a car recently and have had the buyers ask to see the spare key."
Anyone with any information at all should contact the Herts Police non-emergency number 101.