The Prime Minister has announced what is being billed by Downing Street as the biggest programme of investment in the UK Armed Forces since the end of the Cold War.
In a speech to the House of Commons on Thursday, Boris Johnson has set out a £16.5bn increase to the Ministry of Defence's (MOD) budget over the next four years.
The increase is on top of a promised 0.5% annual increase in real terms.
Mr Johnson told the House of Commons the era of cutting the UK’s defence budget must end.
"For decades, British governments have trimmed and cheese-pared our defence budget and if we go on like this, we risk waking up to discover that our armed forces, the pride of Britain, have fallen below the minimum threshold of viability," he said.
"Once lost, they can never be regained. That outcome would not only be craven, it would jeopardise the security of the British people, amounting to a dereliction of duty for any prime minister.
"I have decided that the era of cutting our defence budget must end, and it ends now."
The Government says the extra spending could be as high as £24.1 billion over the four years, when added to the Conservatives' manifesto commitment of the 0.5% uplift.
The Prime Minister said the total additional spending over the next four years will raise the UK's defence outlay as a share of GDP "to at least 2.2%".
He added that the international situation is "now more perilous and intensely competitive" than at any time since the Cold War, as he outlined the need to protect the UK’s "lifelines".
Mr Johnson continued, saying the UK could "hope for the best" and ignore the threat of terrorism and hostile states, but warned: "We might get away with it for a while before calamity strikes, as it surely would, or we can accept that our lifelines must be protected but we’re content in our island and leave the task to our friends.
"My choice... is Britain must be true to our history, to stand alongside our allies, sharing the burden and bringing our expertise to bear on the world’s toughest problems.
"To achieve this we need to upgrade our capabilities across the board.
Boris Johnson also committed to renewing the UK’s nuclear deterrent as well as increasing shipbuilding in the UK.
"We’re going to use our extra defence spending to restore Britain’s position as the foremost naval power in Europe," he said.
Labour Party leader Sir Keir Starmer said the defence spending statement by Boris Johnson shows the Government still lacks a "clear strategy" for Britain in the world.
"This is a time of huge global uncertainty," he said.
"This statement shows the Government still lacks a clear strategy or a coherent vision for Britain in the world, or any idea how the promises the Prime Minister makes will actually be delivered."
Number 10 says the increased funding will allow the Government to invest in cutting-edge technology, positioning the UK as a global leader in domains such as cyber and space, as well as addressing any current weaknesses.
A new agency dedicated to Artificial Intelligence (AI) will also be announced by the Prime Minister, in addition to the creation of a National Cyber Force and a new Space Command that the Government claims will launch its first rocket in 2022.
In total, £5.8bn will be spent on military research and development (a £1.5bn increase), Number 10 said.
There will also be a commitment to invest further in the Future Combat Air System.
The MOD says the boost will also help fund an expansion of Royal Navy vessels creating what the department describes as the biggest surface fleet of modern warships in Europe.
Its aim is to cover "all aspects of international policy from defence to diplomacy and development", with the prime minister describing it as the "biggest review of our foreign, defence, security and development policy since the end of the Cold War".
Final conclusions from the Integrated Review are now due to be published next year.
The £16.5bn extra in the MOD's budget over the next four years is also expected to cement the UK's position as the largest defence spender in Europe and the second-largest in NATO, according to Number 10.
"This is excellent news for defence, and provides us with the financial certainty we need to modernise, plan for the future and adapt to the threats we face," Defence Secretary Ben Wallace said.
Mr Wallace added the investment demonstrated the PM's "recognition" of the importance of defence to the "resilience" and "reputation" of the UK.
"Next year represents a huge opportunity for this country, and defence will be at the forefront of creating the jobs and business opportunities that will help us build back from the pandemic," Mr Wallace said.
More detail will be set out in the coming months for the department’s "ambitious agenda for defence", he added.
Shadow Defence Secretary John Healey MP said the announcement "signals a welcome and long overdue upgrade to Britain’s defences after a decade of decline".
He said: "Since 2010 the size of the Armed Forces has been cut by a quarter, defence spending was cut by over £8 billion and the defence budget has a £13 billion black hole.
"The UK has the know-how to be a global leader in new domains such as cyber and space, as long as Government plays its part. "As the MOD prepares the full conclusions of the Integrated Review, ministers must not repeat the mistakes of the last two Conservative defence reviews.
"High tech weapons systems are essential but highly-trained British troops are indispensable, so forces personnel must be at the heart of the review."
There had been a big question mark over defence funding after Chancellor Rishi Sunak announced he would be reigning back on plans to set a multi-year budget amid the COVID-19 crisis.
MPs from a number of parties had campaigned for an exception to be made for defence, as the Armed Forces continued to tackle emerging threats and handle lengthy procurement processes for kit worth billions of pounds.
Conservative MP and Chair of the Foreign Affairs Committee, Tom Tugendhat, welcomed the decision.
He told Forces News: "We don't just need to maintain the kit we've got; we actually need to upgrade it. In that upgrade we can transform Britain.
"If we have a solid predictable three or four-year time horizon, we can change the way we build ships, we can therefore change the way they are costed and we can also share that technology and cheaper cost with allies and friends around the world, changing the nature of alliances.
"That's why this matters so much, it's all about transformation, it's all about technology and it's all about partnership."