Kemi Badenoch has said Sir Keir Starmer is "all mouth and no trousers" as she called for the long-delayed defence investment plan to be published next week.
Labour ministers had said the investment plan would be published last year, but have since refused to set out a timeline amid reports of a dispute between the Ministry of Defence and the Treasury.
Last month, Defence Secretary John Healey denied reports the Treasury was causing the delay, insisting Chancellor Rachel Reeves had already given the defence budget an additional £8billion.
The Tory leader, who on Saturday will speak at the London Defence Conference, said the failure to publish the plan was a scandal.
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She said: "Keir Starmer talks about wanting to spend 3 per cent of GDP on defence.
"Last year his Government published a Strategic Defence Review explaining why we need it.
"But here we are nine months later, and we still haven’t seen the accompanying defence investment plan.
"It was promised last autumn, it’s now April. This is a national scandal."
Kemi Badenoch said her party would take money from vanity green projects to fund defence
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Mrs Badenoch said the Government had "no idea" how to fund the plan.
The Conservatives have said they would reinstate the two-child benefit cap in order to better fund defence.
The party would also take £17billion from Energy Secretary Ed Miliband's "vanity" green projects.
Sir Keir has pledged to spend 2.5 per cent of gross domestic product on core defence by 2027.
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Sir Keir Starmer is currently at a military base in Saudi Arabia to bolster the ceasefire
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He has also pledged to meet the Nato obligation of 5 per cent by 2035, with 3.5 per cent spent on core defence and 1.5 per cent on security infrastructure, which could include mending roads.
But Mrs Badenoch has said rearming Britain cannot wait until after the next general election.
She said if a joint plan was agreed, she would "put party interests aside" to fund defence, adding that Conservatives would support defence funding on a three line whip.
Responding, Defence Minister Luke Pollard said the Tories "hollowed out" the armed forces while in Government.
Defence Minister Luke Pollard said the Tories 'hollowed out' the armed forces while in government
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In successive Conservative governments between 2010 and 2024, real-terms defence spending fluctuated.
It fell by 22 per cent by 2016/17, after which it steadily rose, recently returning to 2010 levels.
Mr Pollard said: "If Badenoch was prime minister, Britain would be at war in Iran without a plan - she is completely out of her depth.
"Badenoch should start with an apology for the mess they left behind, instead of taking pot shots."
Sir Keir is currently in Saudi Arabia and visited Taif Airbase to help cement the uneasy ceasefire between the US and Iran.
He also met with the Crown Prince of Saudi Arabia, Mohammed Bin Salman, to "give shipping the confidence" to pass through the Strait of Hormuz.
A group of retired military leaders, former ministers and security advisers, last month said Britain was facing a "1936 moment" and urged the Prime Minister to double core defence spending.