Britain will move to "war-fighting readiness" and every citizen must play their part, said as he unveiled a battle plan for the nation's defence.
The Prime Minister said the long-awaited strategic review (SDR) would make the UK "safer and stronger, a battle-ready, armour-clad nation" amid
Speaking at a BAE Systems' shipyard in Govan, in Glasgow, Mr Starmer said the front line "is here" - with threats facing the UK "more serious, more immediate and more unpredictable" than any time since the Cold War.
In an ominous warning, Lord Robertson, who co-wrote the Strategic Defence Review, said Britain's enemies "are at our door". "Cyber attacks, assassinations, election interference, disinformation campaigns" are all activities that are already happening," he told LBC.
The ex-Defence Secretary said: "Our enemies are at our door. They're already there. And when you've got a question about welfare or warfare, you know, what happens if a cyber attack stops our from working? What if it turns our power off?
"What happens if it stops the cars, the police cars in the street? These things are real and alive. And some cases are actually happening today." He warned: "Tomorrow's war could be much nastier, much crueller, much more brutal."
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His warning came as the PM faces after he refused to spell out when his ambition to ramp up defence spending to at least 3% of GDP would be met.
The PM promised in February to hike the amount the country spends on defence to 2.5% of economic output by 2027 - with an ambition to hit 3% in the next Parliament. But chief Mark Rutte has already said that allies need to spend considerably more than 3%.
Paul Johnson, from the respected IFS think-tank, said: "Bluntly, it really does seem to me that the only choice that is available, if we're going to go through all of those things, is some really quite chunky tax increases to pay for it."
Lord Dannatt, former head of the British Army, said that vague commitments to defence spending "does not cut the mustard".
He told Times Radio: "It's a little bit like saying in 1938 to Adolf Hitler, please don't attack us till 1946, because we're not going to be ready. Well, frankly, if we'd behaved like that, we wouldn't be speaking English this morning, would we?"
The government has accepted all 62 recommendations in the review, which include building extra attack submarines, six new munitions factories, £15billion on nuclear warheads and thousands of new long-range weapons. The plans will help the army become "10 times more lethal", with greater use of AI and drones.
The will move to "war-fighting readiness" and cash will be ploughed into defence to create jobs across the country. But the SDR laid bare how the military had been hollowed out under the Tories.

It found that the armed forces were not currently fit to fight or , with inadequate weapons stockpiles, recruitment issues and problems with morale. The PM said Britain must prepare for war if it wants to avoid it.
He said: "Every part of society, every citizen of this country, has a role to play because we have to recognise that things have changed. In the of today, the front line, if you like, is here.
"The threat we now face is more serious, more immediate and more unpredictable than at any time since the Cold War. We face war in Europe, new nuclear risks, daily cyber attacks, growing Russian aggression in our waters, menacing our skies.
"Their reckless actions driving up the cost of living here at home, creating economic pain and hitting working people the hardest. A new era in the threats we face, demands a new era for defence and security, not just to survive in this new world but to lead."
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Mr Starmer said he was "100% confident" that the blueprint can be paid for. Defence Secretary John Healey, who last week said he expected to hit the target by 2034, last night told MPs: “I am totally confident that we will meet the ambition of 3% in the next Parliament."
But questions have been raised about a recruitment and retention crisis on the frontline. Mr Healey said the SDR will mark an end to an era of "hollowing out" of the armed forces.
He told MPs he wants to see the British Army rise to at least 76,000 full-time soldiers in the next Parliament - 2029 to 2034. He told MPs: “For too long our army has been asked to do more with less. We inherited a long-run recruitment crisis, 14 Tory years of cuts to full-time troops.
"Reversing this decline will take time, but we are acting to stem the loss now and aiming to increase the British Army to at least 76,000 full time soldiers in the next Parliament. For the first time in a generation, we are a Government who want the number of regular soldiers to rise."
Mr Healey was challenged by MP Zarah Sultana over how the Government was finding money for defence at the same time as “slashing disability benefits, keeping millions of children in poverty through the two-child benefit cap and cutting winter fuel support for pensioners”.
“The first duty of any government is to defend the country and keep its citizens safe,” he replied. “We invest in defence in order to deter and prevent a war that brings such extreme human and economic costs.
“If we can’t defend the country, where will we be with an without power, with submarine cables that mean data doesn’t work? Strong national security is fundamental to a stable economy, a strong society.”
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