Boris Johnson has used his first intervention in the election campaign to mount the Conservatives’ most personal attack yet on Jeremy Corbyn.

Writing in The Sun, the foreign secretary accused the Labour leader of being a threat to the UK and described him as a “mutton-headed old mugwump”.

He accused Mr Corbyn of being reluctant to use lethal force, opposing nuclear weapons and campaigning against Nato.

Labour said Mr Johnson was “delusional” and Brexit will hurt the UK’s standing.

The Labour leader has insisted he supports Britain’s armed forces.

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In an interview on Sunday, Mr Corbyn said he would not recall 850 British troops sent to Estonia as part of a Nato deployment on Russia’s eastern flank – one of its largest in decades – but also wanted better relations with Moscow.

He said he was opposed to the “first strike” use of nuclear weapons and did not believe the renewal of Trident was a solution to the world’s problems.

Labour, however, has insisted that it remains committed to keeping the UK’s deterrent in its current form and the pledge will be in its manifesto.

Analysis

By James Landale, BBC diplomatic correspondent

So far in this election campaign, Boris Johnson has had an unusually low profile.

So low in fact there were claims he had been deliberately sidelined by Downing Street.

Well, not any more.

In a speech in London, the foreign secretary claimed that the leadership of Theresa May would keep Britain safe. And in an article for the Sun, he argued that the leadership of Jeremy Corbyn would not.

This campaign may have many weeks to go but it has already got personal, deliberately so.

In a deeply personal attack on Mr Corbyn, Mr Johnson said people did not realise “the threat” posed by the Labour leader.

With Mr Corbyn as prime minister, he suggested Britain would be ill-equipped to deal with an assertive Russia, North Korea’s “semi-deranged regime” and so-called Islamic State, which he described as “evil Islamist death cult”

“He seems to have no grasp of the need for this country to be strong in the world,” he said.

Framing the election as a straight choice between Mr Corbyn and Theresa May, he said the former would be “calamitous” for Brexit given the confusion in Labour ranks over its policy.

“Corbyn’s approach would be a recipe for paralysis and uncertainty – and for Britain to get totally stiffed in the negotiations.”

‘Out of hiding’

In a more restrained speech to foreign diplomats in London on Wednesday, he said the Conservative government was committed to “upholding the country’s values and strengthening Britain’s national interests” around the world.

The snap election on 8 June, he believed, would be a “source of continuity, certainty and stability”.

Shadow foreign secretary Emily Thornberry said Mr Johnson – a key figure in the Vote Leave campaign in the EU referendum – had played his part in the “greatest diminution of British influence on the world stage in a generation”.

“It seems Boris Johnson has finally been allowed out of hiding, on the condition he only talks delusional nonsense.

“He talks up a fantastical vision of Britain as a global trading nation, yet he and Theresa May are putting at risk our trade with the EU – by far our largest trade partner – by threatening an extreme Brexit where we crash out on WTO terms.”

“With his crass and offensive remarks Boris Johnson has single-handedly damaged Britain’s chances of getting a good deal with the EU.”