Essex Police is advising motorists not to pull over if requested to do so by anything other than a marked police vehicle.
The warning follows a second theft between junctions 28 and 27 on the anti-clockwise M25 at around 2.10pm on Monday, July 25.
In similar circumstances to a theft which occurred on the M11 on Saturday, July 23, a silver Ford Mondeo equipped with blue flashing light requested a grey Volkswagen Transporter van to pull over.
Three male occupants purporting to be police officers made the driver get out of the vehicle. No weapons were seen but one of the men had handcuffs.
Two of the men then got in to the van and both vehicles drove off. The driver did not get the index number of the Mondeo. The stolen vehicle was registration RE16 UCV.
On Saturday, July 23, between junctions seven and eight on the M11 at Hatfield Heath a white Mercedes Sprinter van was stolen after four men in a silver Ford Mondeo stopped it purporting to be police officers. One of these men was armed with a firearm. This Mondeo was registration LO62 FOU and the stolen Mercedes van KR60 NHZ. The two occupants of this van were also left at the side of the road unharmed.
DCI Stuart Smith from the Kent and Essex Serious Crime Directorate, who is leading both investigations due to their similarities, 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.
“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 use a hands-free to call 999 immediately to verify whether the vehicle and its occupants are genuine.
Anyone with any information about either of these incidents is asked to contact Essex Police on 101.
The police has reviewed its guidance since this story was first published, click here to read more.