Boris Johnson has slammed Nigel Farage as a threat to national security, claiming he had "serious concerns" about Reform UK's approach to the economy and to national security.
"My concerns, and they are serious concerns, are about the approach to the economy of the Reform Party and the approach to our national security," the former Prime Minister told Harry Cole Saves the West.
"I think both of those will be severely tested in the years ahead, and people want serious answers, and I think that the answers are going to be Conservative answers."
When quizzed on whether Mr Farage was a threat to national security, he said: "I'm putting it in [diplomatic] speak right - I have serious anxieties."
Mr Johnson added that the "Reform gang" was "on record as saying that Nato provoked Putin’s aggression".
He said he had "confidence in the Conservatives, and I have confidence in the energy and combativeness of Kemi", despite a recent wave of defections to Reform UK.
The former Tory leader’s attack on Mr Farage’s party follows a series of former Conservatives, including allies of Mr Johnson, abandoning the party to join its insurgent rival in recent weeks.
Mr Farage has also sought to highlight immigration numbers under Mr Johnson's administration, which he describes as "the biggest betrayal in modern political history".
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Boris Johnson said he has 'serious concerns' about Reform's approach to the economy and national security
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Speaking at a press conference on Monday, Mr Farage unveiled his party's most radical plan yet to solve Britain's migration crisis, promising to reverse the "Boriswave" if he won the next General Election.
Mr Farage told reporters: "In particular, what we’re focusing on this morning is the ‘Boriswave’ after his huge victory in 2019.
"And I think the millions that came in the years of his premiership represent the greatest betrayal of democratic wishes, certainly in anyone’s living memory.
"This is not what Brexit voters wanted, and it’s certainly not what any Conservative voter wanted from 2010 onwards."
Nigel Farage made reference to the 'Boriswave' while unveiling his party's latest plans to tackle Britain's migration crisis on Monday
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He added: "Far too many that have come don’t work, have never worked and never will work.
"The ability to bring dependants of all kinds, and when you realise that most that come are very low-skilled, and on very low wages, you start to get a very, very different picture.
"In fact, you start to get a massive benefits bill."
Reform said it would axe the right of migrants to apply for indefinite leave to remain (ILR), ban anyone who is not a UK citizen from claiming benefits, and force those applying for UK citizenship to renounce other citizenship.
The party estimates about 800,000 people will be eligible for ILR in the next few years, after they arrived between 2020 and 2024.
It claimed its plans would save around £234billion.