Foreign secretary ridicules Labour leader and questions his ability to deal with North Korea in first incursion into election campaign

Boris Johnson has made his first intervention in the election campaign to accuse Jeremy Corbyn of being a security threat and not just a “mutton-headed old mugwump”.

After speculation that the Conservative party was hiding the foreign secretary away during the campaign, Johnson used an article in the Sun to claim that the Labour leader had “no grasp of the need for this country to be strong in the world”.

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Johnson used a combination of blunt and characteristically idiosyncratic language to ridicule Corbyn’s leadership credentials.

He claimed the electorate should not be fooled by Corbyn’s “meandering and nonsensical questions”. He added: “They say to themselves: he may be a mutton-headed old mugwump, but he is probably harmless.

“It is absolutely vital for Britain’s security that we have the strong, stable and decisive leadership of Theresa May.”

The foreign secretary continued the theme in an interview with BBC Radio 4’s Today programme in which he repeatedly attacked the Labour leader.

He said: “I do think there is a slight risk that people won’t detect the threat that is hidden behind this innocent air of Islingtonian herbivorousness. There is a real risk that the government of a very great country could be handed over to a guy who has been hostile to Nato all his political career ... who would disarm us of nuclear weapons, and a guy would has said he would not advocate a shoot-to-kill policy in the event of an Islamist terrorist putting innocent people’s lives at risk.”

But Johnson added: “I do want to apologise to mugwumps everywhere.”

In his article Johnson questioned how Corbyn would respond to the “semi-deranged” regime in North Korea. And he warned that the UK would be “totally stiffed” in Brexit negotiations if Corbyn were dealing with Brussels.

“Where is Corbyn on any of these issues?” Johnson asked. “Just when Kim Jong-un is on the verge of acquiring a really deadly nuclear weapon, Britain could be on the verge of acquiring Jeremy Corbyn as PM – a man whose brilliant idea of a nuclear deterrent is that we should send our nuclear subs to sea with no nuclear missiles aboard.”

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

Johnson delivered a more measured version of his article at the lord mayor’s banquet at the Mansion House on Wednesday night. He said: “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.”

Labour accused Johnson of using private school jeering.

Responding to the claim that Corbyn was a mugwump, John Healey, the shadow housing secretary, said: “First of all I had to look that up. I think this is Boris Johnson feeling left out of the election campaign and it is the sort of look-at-me name-calling that you would expect in an Eton playground. I don’t want to sink to that sort of level.”

Johnson’s remarks appeared designed to rebut claims that the prime minister wanted to reduce his exposure to the media and minimise the potential for gaffes during a tightly controlled election campaign. His role has divided Conservative MPs between those who are keen to capitalise on his ability to connect with voters and those concerned about his ability to stay on-message.