Kemi Badenoch has accused Sir Keir Starmer of posturing on the world stage over the Middle East war while failing to rearm Britain.
The Tory leader branded the Prime Minister 'all mouth and no trousers' on defence as he jetted to the Gulf despite playing no part in the ceasefire talks between the US and Iran.
She claimed he has still not published a long-overdue Defence Investment Plan because he had 'no idea' how to pay for his target of spending 3 per cent of GDP on defence by the next Parliament.
Her comments came as Sir Keir, on a trip to Saudi Arabia, insisted the UK's diplomatic efforts were helping to resolve the crisis.
Mrs Badenoch said: 'At a time of war in Europe and war in the Middle East, at a time when those conflicts are affecting every family across Britain, at a time when Britain's place in the world is in flux, our Government literally doesn't have a plan.
'There's no plan for how the Government is going to actually buy the equipment, weapons and munitions. There's no plan for how to enact the SDR [Strategic Defence Review]. There's no plan for rearming Britain.
'We haven't seen the Defence Investment Plan because they have no idea how they are going to pay for it. Keir Starmer is all mouth and no trousers when it comes to defence.'
She will tell the upcoming London Defence Conference that a future Conservative government would reinstate the two-child benefit cap, which Labour has lifted, to fund increased defence spending.
Sir Keir Starmer met service personnel at Taif air base in Saudi Arabia on Wednesday
If they return to power the Tories would also take £17billion from Ed Miliband's 'vanity' green projects to create a Sovereign Defence Fund.
But Mrs Badenoch will warn that rearming Britain cannot wait until after the election, and will call on the PM to 'put party interests aside' and find the money needed now.
'No blank cheques of support. But I can pledge that if we reach agreement on a joint plan, Conservatives will support those measures in Parliament on a three line whip.'
Defence Minister Luke Pollard responded: 'When Badenoch's Tories were last in power, they hollowed out and underfunded our Armed Forces, leaving our brave service men and women exposed. Badenoch should start with an apology for the mess they left behind, instead of taking pot shots.'
However there is still no sign of the Defence Investment Plan, first promised last autumn, amid claims that the Treasury is holding up its publication.
It is unlikely to appear in the next month because of restrictions on Government announcements in the purdah period before May's elections.
And even Labour MP Tan Dhesi, who chairs the Defence Select Committee, warned: 'The lack of a public plan now runs the risk of undermining the UK's ability to play a full and leading role in Nato.'
The row came as Sir Keir – who met UK troops at King Fahd Air Base in Taif, Saudi Arabia – said the UK was working to make the fragile truce permanent and reopen the vital Strait of Hormuz to shipping.
The Prime Minister arrived in Jeddah after a ceasefire was announced between the US and Iran
'We've been convening a number of countries. I now get the opportunity, here in Saudi Arabia and countries nearby in the region, to have these discussions, to co-ordinate our actions and to go forward collectively in pursuit of those two missions,' the PM told broadcasters on Wednesday.
Asked if his relationship with Donald Trump was 'in tatters', after the US President repeatedly insulted the UK's lack of support for his attacks on Iran, Sir Keir insisted: 'I've acted as you would expect of a British Prime Minister, which is by being absolutely focused on what is our national interest, and that's why I've applied my principles and my values throughout.'
Reform UK leader Nigel Farage predicted Sir Keir's trip would go badly, saying: 'He's upset the Americans, he's upset the Cypriots, he's upset the Gulf states, and this level of indecision and chopping and changing your mind is quite extraordinary.'
And Tory peer Lord Kempsell said: 'Starmer has immediately flown to the Middle East to emphasise his crucial role in the US-Iran ceasefire, which he didn't broker, didn't know was coming, didn't participate in shaping and had absolutely no part in whatsoever.'