He writes: “The biggest risk with Jeremy Corbyn is that people just don’t get what a threat he really is.
“They look at him floundering away in the Commons and they say to themselves: Nah – that guy? PM? That’s never going to happen.
“They watch his meandering and nonsensical questions and they feel a terrible twinge of human compassion.
“Have you felt a pang of sympathy for his plight? If so, fight it.”
Johnson then goes on to suggest Prime Minister Corbyn’s anti-military stances would be a threat to the security of the UK with “calamitous” consequences.
He concludes: “The simple fact is that on June 8 there can be only two Prime Ministers: Jeremy Corbyn or Theresa May.”
Shadow foreign secretary Emily Thornberry hit back for Labour, saying: “It seems Boris Johnson has finally been allowed out of hiding, on the condition he only talks delusional nonsense.
“He talks about creating a ‘global Britain’, yet the Tories have overseen the greatest diminution of British influence on the world stage in a generation.
“With his crass and offensive remarks Boris Johnson has single-handedly damaged Britain’s chances of getting a good deal with the EU.
“And after his broken promise of £350m a week for the NHS, why should anyone believe a word he says?”
Johnson has kept a low profile so far in the election campaign even leading some to suggest he had been sidelined although a string of media appearances on Thursday morning as the government’s main spokesman on the election swiftly put that rumour to bed.
He also spoke at the Lord Mayor’s Banquet at Mansion House in London on Wednesday, saying: “In recent years we have seen an increase in the global tally of deaths from wars. We and our allies face threats from countries with a nuclear weapons capacity, and from those trying to acquire that capacity.
“For the first time for many years, some countries are trying to change European borders, not by agreement, but by force. And, as we have seen across Europe in recent months, we face a continued battle against terrorism and the hateful ideology of Islamic extremism.
“These are uncertain and unstable times. Britain needs to help manage them in a serious and clear-sighted way.”