Sir Keir Starmer said the UK had to be prepared for war to be able to deter potential enemies as he set out the £15 billion defence investment plan (Dip).
The Prime Minister acknowledged some other areas of Government spending would be slashed in order to fund defence, with road and energy projects scrapped to pay for the military.
Sir Keir said: “When the world is arming and aggression is rising, the best way to avoid war is to prepare for it.
“The best way to defend is to deter, to have the strength to make your adversaries think again before they act.
“And that is what we are doing.”
Annual defence spending will increase from £54 billion when Labour came to power to £80 billion by 2029, Sir Keir said.
The plan reversed the “corrosive hollowing out” of the armed forces, he said in a speech at a drone company in Berkshire.
Chancellor Rachel Reeves, who was also at the event, said the extra money for the Dip had come from “reprioritising spending” across Government.
Sir Keir set out where the trade-offs had been made, with the money found by raiding Whitehall capital budgets, as he acknowledged there were “no easy answers”.
He said if the Government had chosen to “slash funding to our public services in favour of defence” then “we would be fundamentally weaker as a nation, more fractured as a society, less able to defend ourselves when our enemies prey on social division”.
The Dip would not take resources away from day-to-day spending on frontline services such as health and education, he said.
“Instead, it is funded by reallocating spending from across Government departments, reallocating capital budgets by one penny in every pound, while still maintaining public investment at the highest sustained levels since the 1970s.”
He added: “Therefore, some capital projects, for example on roads and energy, which are important but not immediately vital, will no longer go ahead as planned.”
The Dip, which was originally due to be published last year following the strategic defence review, was repeatedly delayed by Whitehall wrangling over the funding.
John Healey quit as defence secretary earlier this month, claiming that he had only been offered £13.5 billion for the Dip.
His successor Dan Jarvis appears to have wrung some extra money out of the Treasury and his Cabinet colleagues in order to reach the £15 billion figure, although defence experts still believe this is not enough.
The announcement of the Dip could be one of the final acts of Sir Keir’s premiership as he attempts to secure his legacy before handing over power, which could happen within weeks if – as expected – Andy Burnham is the only candidate to replace him.
Sir Keir said he is “absolutely certain” his successor as prime minister will build upon the Government’s increases in defence spending.
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