AMERICA has warned it can’t plug gaps in Britain’s hollowed out armed forces in the event of all out war.
General Randy George, head of the US Army, insisted Britain must be self-sufficient.
He stressed the need for vast ammo stockpiles to keep the big guns firing in a drawn out conflict.
Speaking on a UK tour, he said Britain must be able bring all the weapons it needs to a fight.
He warned it would “be difficult for another army” – such as the US – to help if the flag goes up.
He said: “What I would want out of the British army? You have to have magazine depth.
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“Whatever formations you have, do you have all the ammunition you need to sustain yourself over a long fight?”
Crucially, he added: “If you are going to come to the fight make sure you have every capability on the battlefield because it will be difficult for another army to provide that.”
Capabilities include air include air defence missiles, surveillance drones and electronic warfare devices to protect frontline troops and supply chains.
Britain would rightly expect to fight alongside Nato allies in the event of a major conflict.
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But Gen George's message to Britain's Top Brass was that they can't expect America to do their heavy lifting and logistics.
Speaking at the Rusi think tank in London, he said: “We need to ensure we are prepared to warfight at scale.
"One of the lessons from Ukraine is we can have the best weapons in the world, but if we run out of bullets it won’t matter a damn.
“We are going to have to have bullets and ammo at scale.”
But he warned that getting “beans and bullets where they need to go” will be harder than ever before due to the threat from long missiles and drones.
He also said it was vital to protect air fields and bases were protected from "air, space and missile threats".
He said the world was as dangerous as he had ever known it in his 36 years of army service.
Britain has cut its Army to the smallest size in 300 years and former defence ministers including Ben Wallace warned it has been hollowed out by decades of under investment.
The UK has 73,000 fully trained soldiers – 20,000 fewer than when the Conservatives came to power in 2014.
The Ministry of Defence vowed to double its ammunition budget to more than £2bn a year over the next six years.
And government pledged to increase defence spending to 2.5 per cent of GDP by 2030.
But critics accused Grant Shapps, the defence secretary, of manipulating statistics to claim it was a £75 billion increase in the defence budget.
Privately Top Brass admit the British army currently unable unable to meet Gen George’s demand, “to be ready to war fight at scale”.
Gen George compared the current threats to the build up to World War Two when Winston Churchill warned about Nazi Germany’s rearmament.
He said: “You have to wonder today if we aren’t facing a Churchillian moment
“There are flashing red lights all around us forcing us to make difficult choices.”
The Sun revealed last year that Britain’s ammunition stockpiles would run out in an afternoon of a shooting war with Russia.
Gen Georce said wars "always last longer than we want" and normally come down to a "close fight" between soldiers.
He said: "Even in an age of nuclear weapons there’s nothing like a professional disciplined army at the gates.
"Wars walays last longer than we want. They usually come down to the close fight, whether in the streets of Falluhjah or on the moutnain tops of the Falklands."
He refused to be drawn on whether Russia, China or another country presented the greatest threat.
General Sir Richard Barrons, a former commander of the UK’s Joint Forces Command, said Brit troops were nicknamed the “borrowers” by US forces in Iraq and Afghanistan.
He said: “The point that George is making is that the British Army has to turn up able to do its job, not half a job.
"That means enough people, equipment, that is good enough and works, trained to the right level, and supported with intelligence, logistics, medical capability, ammo, and money. “
He said British troops in Iraq and Afghanistan turned up with headquarters and units “with plenty of opinions but not enough stuff to actually fight”.
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The top US general in Nato, known as Supreme Allied Commander Europe, recently warned he no longer saw Britain has having a Tier One army.
Gen Barrons added: “We should see his comments also in the light of the certainty that whoever wins the US election, the US is no longer going to pay for 70 percent of Nato, now that Europe has grown so wealthy, compared to 1947.”