Keir Starmer branded Rishi Sunak “insulting and childish” during the first head-to-head debate of the election campaign.
The pair clashed repeatedly during the hour-long ITV programme.
But it was the prime minister’s refusal to deny that he believes the Labour leader is a threat to national security which saw tempers boil over.
Presenter Julie Etchingham asked Sunak: “You raised a question there very specifically at the end of that statement ... about our security or otherwise under the potential leadership of Keir Starmer.
“Do you really think the man standing next to you is a threat to our national security?”
The PM replied: “I don’t think the Labour Party can be trusted to keep this country as safe as the Conservatives.”
Starmer could be heard saying “shocking” as Sunak continued: “The world is a more dangerous and uncertain place than at any point since the end of the Cold War.
“I’ve made the decision to invest more in our defence, taking it up to 2.5% of GDP, the Labour Party have not matched that. And worse than that, the person who would be deputy prime minister under Keir Starmer does not believe in our nuclear deterrent, the ultimate guarantor of our security.
“So you can trust the Conservatives with out security, and my track record and our commitments prove that.”
Starmer replied: “This is shocking. Before I was a politician, I was the Director of Public Prosecutions, I was working on national security. I was dealing with terrorist plots.
“One of them that we prosecuted was to take down seven aeroplanes as they were halfway across the Atlantic. They were going to be blown up - that is one of the cases that I prosecuted, knowing how important it is to keep everybody in this country safe and sound.
“For the prime minister, who at the time was making money betting against the country during the financial crisis to now say that somehow national security is somehow safer in his hands is insulting and childish.”
Both men also clashed over tax, with Sunak accusing Labour of plotting to put them up by £2,000 per household after the election - a claim dismissed as “absolute garbage” by Starmer.
Meanwhile, the pair also went toe-to-toe over immigration and the NHS, but neither managed to land a killer blow.
A snap YouGov poll after the debate found that 51% believed Sunak had won compared to 49% who though Starmer had - effectively a dead heat.
The survey of 1,657 people showed that Starmer was deemed to be more trustworthy, likeable and in touch with ordinary people, with Sunak seen as slightly more prime ministerial
Starmer also won the sections on the NHS, education and the cost of living, while Sunak came out on top on immigration and tax
But with the Tories still miles behind Labour in the polls, the prime minister failed to claim the comprehensive win he needed to get his party back into the race.