A driver was robbed of his car after two men posing as police officers pulled him over.
The two men were in a blue people carrier with blue flashing lights on the roof when they followed the white Audi S3 and waved the man down on Stonards Hill, Epping, at about 9.30am yesterday.
The driver of the Audi got out to ask what was wrong and the men told him to hand over his keys.
One of the men got into his car, turned it around in Stonards Hill and drove off, followed by the other thief driving the people carrier.
They sped off towards east London through Epping High Street, where one of them is believed to have thrown a mobile phone out of the Audi window.
Police believe both cars entered the forecourt of the Shell garage near the Wake Arms roundabout between Epping and Loughton and the people carrier was following the Audi when they left.
The victim was unhurt but badly shaken.
Both suspects are described as white and about 5ft 11ins tall.
One was about 20 and was wearing a dark woollen hat, a dark hoodie and grey jogging bottoms. The other was wearing dark clothes.
Police have advised anyone who is stopped by someone claiming to be from the emergency services to ask for identification, which they will always carry.
Witnesses should call Detective Sergeant Steve Jones on 101.