BORIS Johnson has been rounded on by some of the UK’s top historians for claiming the Roman Empire’s demise was primarily due to “uncontrolled immigration”.

Standing in the Colosseum, the Prime Minister likened the impact of unrestricted climate change to the fall of the ancient empire.

“Civilisation could go backwards and history could go into reverse,” he said in an interview with Channel 4 News.

“Here we are in the Colosseum of Vespasian, the Roman Empire, they weren’t expecting it and they went into reverse. We had a Dark Ages. It’s important to remember things can get dramatically worse.”

Johnson was in the Italian capital for a summit of the G20 leaders ahead of the crucial COP26 climate change talks.

In a round of broadcast interviews he acknowledged that reaching an agreement that would keep alive the hope of restricting global warming to 1.5C would be difficult. But he used the dramatic historical backdrop to underline the dire consequences for the whole of humanity if they failed.

“If you increase the temperatures of the planet by four degrees or more as they are predicted to do remorselessly, you’ll have seen the graphs, then you produce these really very difficult geopolitical events,” he told Channel 4 News.

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“You produce shortages, you produce desertification, habitat loss, movements … contests for water, for food, huge movements of peoples. Those are things that are going to be politically very, very difficult to control.”

He added: “When the Roman Empire fell, it was largely as a result of uncontrolled immigration. The Empire could no longer control its borders, people came in from the east, and we went into a Dark Ages.

“The point of that is to say it can happen again. People should not be so conceited as to imagine that history is a one-way ratchet.”

Those claims were widely rubbished by leading historians on social media.

Author Mik Duncan commented: “On the list of things that caused the Western Empire to collapse, I would gently suggest the Prime Minister investigate the role of incompetent and cowardly leaders at Court clinging to self-destructive xenophobia as being significantly more instructive.”

Public historian and broadcaster Greg Jenner added: “This is baaaaad history, used for nasty ends. Our PM should know better, but it's hardly surprising from him.”

Dr Eleanour Janega, a medieval historian, wrote: “The thing is that everything Johnson has just said is incorrect, but you know that. We all know that, and he does these things on purpose in order to distract from the fact that his government is responsible for the needless deaths of thousands of people. I am tired of this.”