Boris Johnson accused Jeremy Corbyn of siding with the UK's "enemies", as world leaders arrive in Britain for a two-day Nato meeting.
The Conservative Party leader launched a stinging attack on Mr Corbyn, just days after the London Bridge terror attack.
In an interview with the Sun newspaper ahead of Nato's 70th anniversary, a military alliance created during the Cold War, Mr Johnson said the UK's allies were "very anxious" about the prospect of Mr Corbyn being elected, accusing him of being "naive" to terror threats.
The Prime Minister even suggested the likes of the US, Canada, Australia and New Zealand – the nations that, with Britain, make up the so-called Five Eyes intelligence-sharing agreement – could stop working closely with the UK if Mr Corbyn became PM.
“Every time he has the chance, he sides with our enemies,” Mr Johnson told the Sun.
“A lot of our allies, particularly the Five Eyes, are very anxious about any future collaboration.
“It is absolutely not a Tory scare story.
"They have said precisely this.”
A Labour spokesperson hit back at the claims, saying the Conservatives were trying to "keep people safe on the cheap".
A party spokesperson said: "Jeremy Corbyn has consistently made the right calls in the interests of peace and security at home and abroad and will do whatever is necessary and effective to keep the British people safe.
“Real security doesn’t just come from strong laws and intelligence, it also comes from effective public services that have the funding they need.
"You can’t keep people safe on the cheap.”
Mr Johnson's remarks came just hours after Donald Trump's Air Force One landed at Stansted Airport, with the US President in the UK for the two-day meeting.
“He would be seething at his death, and his life, being used to perpetuate an agenda of hate that he gave his everything fighting against,” he told the Guardian.
Mr Johnson told supporters at a Tory rally in Colchester on Monday evening that, if put back in Downing Street, he would be “stopping the early release, the automatic early release of serious and violent offenders and terrorists”.