Boris Johnson has cemented his intervention in the General Election campaign with a savage attack on Jeremy Corbyn, describing him as a "benign herbivore".
His colourful assault on the Labour leader suggests voters are tempted to feel compassion when they see his "meandering and nonsensical" performances.
But the Foreign Secretary writes in The Sun that voters should not feel sorry for the "mutton-headed old mugwump" as, he says, Mr Corbyn poses an extreme danger to the country.
He says the Opposition leader's long record as a peace campaigner and his anti-military stances actually mean "the consequences would be calamitous" if he ever became Prime Minister.
"Have you ever thought the Leader of the Opposition is an essentially benign Islingtonian herbivore?" the Foreign Secretary writes. "Have you felt a pang of sympathy for his plight? If so, fight it.
"The biggest risk with Jeremy Corbyn is that people just don't get what a threat he really is."
Mr Johnson concludes his article with a warning on Brexit. "Corbyn's approach would be a recipe for paralysis and uncertainty - and for Britain to get totally stiffed in the negotiations," he writes.
Shadow foreign secretary Emily Thornberry said: "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?"
Shadow housing secretary John Healey told Sky News: "I really think this is Boris Johnson feeling left out of the General Election campaign and this is 'look at me name-calling' and to be quite honest it demeans the position of Foreign Secretary."
He urged Mr Johnson to "debate the policies" and not to "attack the person".
On Wednesday night, at the Lord Mayor's Banquet in the City of London, Mr Johnson strongly endorsed Mrs May's Brexit strategy.
He also praised the Prime Minister's leadership in the war on terror and attacked Jeremy Corbyn's stance on nuclear weapons, tackling terrorists and airstrikes in Syria.
Mr Johnson's election campaign debut came after reports of a battle over his role in the campaign.
Cabinet rivals are said to have claimed he is "toxic" to some Remain-supporting Tories.
But on Brexit, he said: "We have a clear plan for Brexit - as the Prime Minister set out in her Lancaster House speech and in her letter to Council President Tusk - to get a good deal that works for both Britain and our European friends.
"One which enables Britain to work with the rest of the EU in a deep and special partnership, which can end the British question and bring stability to Europe, and which protects the interests of all our citizens on both sides of the Channel.
"And though I have no doubt that the negotiations will be tough and some plaster may fall off the ceiling, I am also sure that Theresa May can pull it off, and usher in a new era of free trade deals."
The Foreign Secretary's speech, which showed every sign of having been vetted by Number 10, was relentlessly on message and stuck to the Prime Minister's election theme of emphasising her strong leadership.
On the War on Terror, he said that in unstable and uncertain times around the world, strong leadership was needed and - in an apparent swipe at Tony Blair - said there had been complacency in the past two decades.
"That is where leadership comes in - clear leadership to navigate this age of uncertainty," he said.
"We are determined to provide that leadership, to give people the security and certainty they need.
"There can be no more important task for a Government than to keep people safe - and we must be prepared to do everything necessary to do so.
"It is why the Prime Minister made it a priority when she took office last year to ensure the renewal of Britain's crucial independent nuclear deterrent and to lead the debate in Parliament.
"It is why she made it clear in the US earlier this year that Britain saw our profound security and defence alliance with the US as part of the bedrock of global security in the modern world.
"And it is why the PM confirmed last week that Britain will continue to spend 0.7% of our gross national income on overseas development.
"Indeed I have seen with my own eyes the good we are doing, the lives we are changing, the women we are educating, with that money, and that is why perhaps this was one of the first pledges the PM renewed in this election campaign."
And echoing a phrase previously used by the Prime Minister in a speech of her own, he said: "So, as we look forward, now is not the time to step back, but to step up.
"Under this Government you know what you are getting. A Government that works to uphold our values. To protect our way of life.
"And to stand strongly in defence of Britain's interests. And leading a country that stands up for its principles."
Mr Johnson also used his speech to attack the Labour leader, who in an interview at the weekend ruled out pressing the nuclear button and killing the leader of IS and said he would suspend airstrikes on Syria.
"We have supported the US in acting against the murder of innocents with chemical weapons in Syria - and we will work to hold accountable those who are responsible," Mr Johnson said.
"We insist on the sovereignty and territorial integrity of all European countries, and refuse to accept Russia's illegal annexation of Crimea."