Two anti-Brexit campaigners say they were pulled over on to the hard shoulder of the M25 and told by police to remove a slogan from their car.
Passenger Peter Cook said the officer was "extremely aggressive" towards them over the phrase "bollocks to Brexit".
"He said it [the slogan] was a public order offence, he insisted we remove the signage on the hard shoulder of the M25," said Mr Cook.
Essex Police said the force was trying to establish what happened.
Mr Cook said the officer stopped them on the motorway near Brentwood at 14:30 GMT on Sunday.
He said the policeman repeatedly shouted at him and the driver during the 40 minutes they were pulled over and "bullied" them into removing part of the slogan.
Arrested development— BollockstoBrexitBus (@BollocksBus) November 24, 2019
Essex Police bullied us into removing "Bollocks" from our car
I am calling the Police Complaints Commission @TheNewEuropean @siandavies36 @guardian@acgrayling@vincecable @joswinson@Anna_Soubry @GuitarMoog pic.twitter.com/kmuvIW8Yqd
The car was eventually moved to Brentwood Services after Mr Cook called 999 over fears for his safety at the roadside.
Some of the wording was then removed before the officer told the pair he would allow them to leave without further action being taken.
Mr Cook, an Independent candidate for Gillingham and Rainham in Kent for the upcoming election, said the policeman had been "bang out of order".
The Independent Office of Police Conduct (IOPC) has been contacted for comment.
An Essex Police spokeswoman said: "We are trying to identify the officer from the video footage to establish the circumstances around what happened.
"In the meantime, we would encourage the driver to get in touch with us."
In November 1977, Nottingham Magistrates' Court ruled "bollocks" was not an obscene word in the case of a record store manager who had been arrested for displaying the Sex Pistols' debut album Never Mind The Bollocks, Here's The Sex Pistols.