The Prime Minister described the demonstrators demanding action on climate change as “uncooperative crusties” who were holding up the traffic.
Mr Johnson was speaking at the launch in London of the third volume of Margaret Thatcher’s biography by the former editor of The Daily Telegraph, Charles Moore.
He said the former prime minister had taken the issue of greenhouse gases seriously long before Greta Thunberg was born.
“I am afraid that the security people didn’t want me to come along tonight because they said the road was full of uncooperative crusties and protesters all kinds littering the road,” he said.
“They said there was some risk that I would be egged. And so I immediately asked the faint hearts in my private office, ‘What would Margaret Thatcher do?’
“If she could take the extraordinary risk of sending a task force halfway around the world through tumultuous seas to recapture the Falklands, I think she would have crossed the road to speak at the Banqueting Hall.”
He added: “I hope that when we go out from this place tonight and we are waylaid by importunate nose-ringed climate change protesters, we remind them that she was also right about greenhouse gases.
“The best thing possible for the education of the denizens of the heaving hemp-smelling bivouacs that now litter Trafalgar Square and Hyde Park would be for them to stop blocking the traffic and buy a copy of Charles’s magnificent book so that they can learn about a true feminist, green and revolutionary who changed the world for the better.”