The UK will increase its defence budget to 2.5 per cent of its gross domestic product by 2027, with Prime Minister Sir Keir Starmer saying Europe cannot be complacent against the Russian "menace".
Addressing the House of Commons, the prime minister said his government was entering a "new era", but reaffirmed the importance of the NATO military alliance.
"Starting today, I can announce this government will begin the biggest sustained increase in defence spending since the end of the Cold War," he told Westminster.
Defence spending is set to increase from 2.3 per cent currently to 2.5 per cent of GDP by 2027, and then 3 per cent later in the decade.
Sir Keir said the rise would equate to an extra 13.4 billion pounds ($26 billion) in spending each year.
The money would come from major cuts to the UK's international aid budget.
On social media, US Defense Secretary Pete Hegseth welcomed the spending rise after speaking with British Defence Minister John Healey.
"A strong step from an enduring partner," Mr Hegseth said on X.
Boost comes amid uncertainty in Europe
Mr Starmer made the announcement a day before he is scheduled to meet US President Donald Trump in Washington DC.
Mr Trump has long urged European nations, including the UK, to increase their defence spending, accusing the US' NATO allies of being too reliant on America's military.
"So this week, when I meet President Trump, I will be clear: I want this relationship to go from strength to strength," he said.
"Strength in this world also depends on a new alliance with Europe."
In recent weeks, European leaders have grown increasingly alarmed at being cut out of talks to negotiate an end to the war in Ukraine.
Earlier this week, the US split with allies and voted with Russia against a UN resolution calling on Vladimir Putin's forces to withdraw from Ukraine.
Mr Starmer said the increase was designed to counter the rise of Russian aggression.
"Tyrants like Putin only respond to strength," he said.
"They've launched cyber attacks on our NHS, only seven years ago a chemical weapons attack on the streets of Salisbury."
"We must stand with Ukraine, because if we do not achieve a lasting peace, then the economic instability, the threats to our security, they will only grow."
ABC/wires
