The secret sensors, fitted to all new and used motors sold by the firm’s dealers, pinpoint the vehicle’s exact location. The firm sold more than 170,000 new cars in Britain alone last year. Mercedes will not say how long it has used the sensors.

And it insists they are only activated in “extreme circumstances” — when finance customers have defaulted on their payments. But it admits sharing car owner information and vehicle location details with third-party bailiffs and recovery firms who repossess the cars.