Essex Police is urging motorists not to pull over if they are requested to do by "anything other than a marked police car" following two van thefts earlier this month.
According to the force, on both occasions a silver Ford Mondeo equipped with blue flashing lights requested a grey VW Transporter van and a white Mercedes-Benz Sprinter to pull over on motorways in the county - the M11 and M25.
Essex Police said in both cases, men driving the Mondeo were dressed in a fake police uniform, with one reportedly carrying a firearm.
In both cases, the van owners were left by the side of the road unharmed, the force said.
"In a direct response to these incidents occurring a direction has been given to our officers that they should not, unless in emergency circumstances, be in an unmarked car and attempt to stop a driver," said DCI Stuart Smith from the Kent and Essex Serious Crime Directorate. "We have taken this decision to safeguard motorists in Essex while these offenders remain outstanding. Our victims have told us that the suspects are purporting to be police officers and are wearing body armour to further enhance this deception in order to steal these vans."
"Anyone who is signalled at to stop by someone in a car which may appear to be an unmarked police is asked not to stop but to call 999 immediately to verify whether the vehicle and its occupants are genuine," he added.