Two men in dark clothing driving an unmarked BMW with blue flashing lights pulled over a driver on the A47 near Wisbech and claimed his car was uninsured - but refused to give police collar numbers.
Officers received a report from a member of the public about being pulled over by an unmarked black 5 Series BMW, with blue flashing lights in the grille.
Police said that "two males dressed in dark clothing with local accents approached the driver stating that the vehicle was showing up as uninsured.
"When the driver asked for the collar numbers of the two males, this was refused. The driver became suspicious of their behaviour, drove home and contacted the police."
Fenland Police have since issued advice to drivers if they find themselves in a similar position.
They said: "If you find yourself in a similar position where an unmarked vehicle with blue flashing lights is indicating for you to pull over, and you are unsure if this is a legitimate vehicle, please wait until you are in a safe, well lit populated area to stop.
"Call the police and give them as much information as possible - they will be able to confirm if it is a police registered vehicle.
"Ask for ID and/or collar numbers - no police officer will refuse to do this."
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here