Boris Johnson will make his first major intervention in the general election campaign by warning that the UK faces a "world of threats" and needs strong leadership, as he address City leaders in London on Wednesday (26 April) evening.

Johnson's speech to the Lord Mayor's Banquet at Mansion House comes amid reports that the bombastic and sometimes off-script foreign secretary is being kept away from the limelight by Conservative strategists.

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The former Mayor of London was the chief cheerleader for the successful Vote Leave campaign during the EU referendum. But Work and Pensions Secretary Damian Green and Defence Secretary Michael Fallon, among other "core team" ministers, have been touring the TV studios for Theresa May.

"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," Johnson will say.

"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."

He will add: "That is where leadership comes in. Clear leadership to navigate this age of uncertainty. We are determined to provide that leadership, to give people the security and certainty they need.

"Because there can be no more important task for a government that 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."

The "safe and stable leadership" claim is at the heart of the Conservative election campaign, with the party continually raising Jeremy Corbyn's anti-nuclear record. With just over six weeks to go before the 8 June vote, the Tories are 23 points ahead of Labour (49% versus 26%), according to the latest poll from Ipsos MORI.

Emily Thornberry MP, Labour's Shadow Foreign Secretary, 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.

"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.

"And after his broken promise of £350m a week for the NHS, why should anyone believe a word he says."