Paul Powlesland held up a blank sign to an officer amid a number of high-profile protests which saw a heckler shout at Prince Andrew and a woman charged after she held up a sign saying “abolish monarchy”.
In the altercation covertly filmed, the barrister says: “Why do you need my details? I was holding up a blank sign, why do you need my details?
The officer replies: “Because you said you were gonna write stuff on it that may offend people around the King.”
Mr Powlesland hits back: “I said I was going to write ‘not my King’ on a sign, you ask for my details.
“Who’s that going to offend?”
The officer responds: “It may offend someone, I don’t know.”
Scotland Yard has been approached for comment.
Mr Powlesland said he was not arrested and managed to talk his way out of it.
He told the Standard: “I’m concerned. This and all instances of people being arrested or threatened with arrest by the police for peaceful free expression against the accession of Charles are concerning.”
He added: “It could get worse. But to be fair to the officer, he wasn’t threatening to arrest me for the blank paper, but if I wrote the words ‘not my King’ on it.”
Legal author the Secret Barrister wrote on Twitter that the officer had misinterpreted the Public Order Act, adding: “Unless the sign if threatening or abusive, no offence is committed. Being merely offensive is not an offence.”
A Scotland Yard spokesman said: “We are aware of a video that appears to show an officer engaging with a member of the public outside Parliament.
“We have been clear that the public have the right to protest and all officers are being reminded of this throughout the ongoing operation.”
The woman, wearing a straw hat, was escorted away from the palace gates after unfurling a hand-drawn sign, proclaiming: “Not My King” on Monday.
She was led away by five police officers and made to stand on the other side of the road.
A witness said she was not arrested and was allowed to continue her protest.
The man was seen shouting at the royals as the procession made its way from the Palace of Holyroodhouse to St Giles’ Cathedral on Monday afternoon.