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.”
Read MorePolitics latest: PM ‘takes pride’ in defence legacy he unveils criticised £15bn plan
Defence funding plan not enough to protect country, says military expert
Heathrow offers therapy to residents over ‘incredibly difficult’ expansion plans
Britain sets out a plan for future defense with a focus on drones and a fight over money
Man given £1k fine for leaving cardboard box next to overflowing bin
Driver in fatal crash escaped prosecution because it didn’t happen on a road
