The UK is reviewing the size of its ammunition stockpiles, Sky News can reveal.

Russia’s invasion of Ukraine has exposed how past assumptions on what would be needed to fight a war were far too small, Whitehall and defence industry sources have said.

They urged Rishi Sunak to boost military spending to fund a massive expansion in total munition stores or else risk no longer being able to support the Ukrainian armed forces at the level needed to sustain their war effort, let alone secure Britain’s own defences.

The sources, however, said they feared the prime minister – a former chancellor – was not “interested” in defence and did not understand the need to rearm with urgency, despite a major land war raging in Europe, because his expertise lay in finance.

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It is “really sobering”, one Whitehall source said, speaking on condition of anonymity to discuss a sensitive topic ahead of the government’s budget next month.

The source claimed that Mr Sunak was even slow to engage with a historic visit to the UK earlier this month by Volodymyr Zelenskyy, Ukraine‘s wartime leader.

He “has no interest in defence and security. Those charged with that in Number 10 can barely get access to him. The President Zelenskyy visit was engaged with late and with only peripheral passing interest. If it’s not domestic or economic, it doesn’t feature”.

A second Whitehall source told Sky News: “He is a financier and simply can’t understand these things.”

A government spokesperson strongly disputed the characterisations and pointed to a previous increase in defence spending as evidence of Mr Sunak’s interest.

“These claims are baseless and untrue,” the spokesperson said.

“It was the prime minister who, as chancellor, agreed the 2020 spending review that provided the Ministry of Defence (MoD) with the largest increase in defence investment since the Cold War.”

The new review into stockpile requirements has been taking place as part of a wider refresh of UK defence and security policy, the sources said.

They warned that any significant uplift in the size would require new funding, which has so far not been guaranteed.

The sources offered a sense of the scale of the expansion they said was called for.

A defence industry source said there needed to be a 100% increase in the number of precision-guided missiles. A second source said the boost needed to be far higher.

It is well understood within the Ministry of Defence and its procurement arm, Defence Equipment and Support (DE&S), that stockpile requirements of ammunition, such as missiles and artillery rounds, are woefully inadequate given the lessons from Ukraine.

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All other members of the NATO military alliance are grappling with the same problem.

Russian and Ukrainian forces are launching artillery shells at each other at a rate not seen since the Korean war – with thousands to tens of thousands of rounds fired daily.

Jens Stoltenberg, the head of NATO, has warned member states that they are in a “race of logistics” to mobilise and expand their defence production capacity.

The British government’s strategy before Russia’s invasion, had been to hold a limited number of warfighting supplies – it costs money to keep in storage things like ammunition and spare parts – and rely on industry to deliver more in a crisis.

However, the sheer volume of equipment needed to sustain a war effort like Ukraine’s has demonstrated that this plan would not work in practice, according to the sources.

The industry source said the government needed to forge a new “special relationship” with the UK’s defence companies and work together to ramp up production – which crucially requires contracts – at the pace of urgency needed.

Defence Secretary “Ben Wallace is doing his part, banging his fist against the table”, the industry source said.

“The Ministry of Defence is saying it needs £8bn to £11bn over the next two years just to keep still. It is all the right rhetoric. But my headline is: where is the contract?”

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The source described long conversations between defence companies and DE&S about the need to replenish weapons stockpiles and expand production lines – but it was taking too long because of uncertainty over funding.

“Let’s stand up and show the world that we take defence seriously,” the source said.

Britain’s has led Europe with supplying arms to Ukraine, such as tanks, rocket launchers and missiles, so Ukrainian troops are better-equipped to fight Russia’s invading forces.

But this generosity has eroded the British Army’s ability to fight, which had already been reduced because of decades of cost-saving cuts since the end of the Cold War.

On Wednesday, the defence secretary told the Reuters news agency that the UK had begun to “warm up” its production lines to replace weapons sent to Ukraine and increase production of artillery shells.

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He said shells could be made fairly quickly but “the key is to make sure that we place the orders, and we’ve started placing those orders over the last 10 months and that starts to sort of warm up those production lines”.

The defence secretary would not be drawn, however, on how talks were going with the Treasury to secure new money for the military in the March budget.

When asked whether he felt Mr Sunak understood what was needed, Mr Wallace said: “I am reassured.”

The government spokesperson said the UK is the biggest defence spender in Europe.

“The prime minister is clear that we will do everything necessary to protect our people, which is why we our armed forces will always have the equipment and capability they need,” the spokesperson added.

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The refresh of the Integrated Review was initially scheduled to be published on 7 March ahead of Jeremy Hunt’s budget on 15 March.

But, as previously reported by Sky News, that date is set to slip because an initial draft of the document failed sufficiently to reflect the impact of Russia’s full-scale invasion of Ukraine on the security landscape and the UK’s military assumptions.

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