The UK government will spend 2.5% of GDP on defence by 2030, Boris Johnson has announced, as NATO's secretary general warned that the alliance must stand up to Vladimir Putin.
Speaking from the NATO conference in Madrid, the prime minister told broadcasters that "we need to invest for the long-term in vital capabilities like future combat air, whilst simultaneously adapting to a more dangerous and more competitive world".
"The logical conclusion of the investments on which we propose to embark, these decisions, is that we'll reach 2.5% of GDP on defence by the end of the decade," he continued.
The sum equates to a further £55 billion.
Mr Johnson said the spending targets set by NATO members a decade ago - with every country pledging to spend 2% of GDP on defence - "were then set for a very different era".
"What we're saying is that we want Jens Stoltenberg, the [NATO] secretary general, to start work on that new target now - and he's agreed to do that."
The PM added that "the cost of freedom" is "always worth paying".
"Unless we get the right result in Ukraine, Putin will be in a position to commit further acts of aggression against other parts of the former Soviet Union, more or less with impunity, that will drive further global uncertainty, further oil shocks, further panics and more economic distress for the whole world," he warned.
Mr Johnson spoke after NATO chief Jens Stoltenberg said the alliance's boost in troop numbers on its eastern flank was to ensure Russia's president does not miscalculate or misunderstand allies' readiness to act.
"We live in the more dangerous world, and we live in the more unpredictable world, and we live in a world where we have actually got war going on in Europe with large scale military operations," he said.
"We also know this can get worse because if this becomes a full scale war between Russia and NATO, then we see suffering damage, death, destruction at this scale, which is much, much worse than what we see in Ukraine."
There had been an ongoing debate back in the UK over whether defence spending should be increased in light of the conflict in Ukraine.
Defence Secretary Ben Wallace had urged the PM earlier this week to increase spending on the UK's armed forces in response to the war.
Responding to the PM's words, a source close to Mr Wallace said: "The defence secretary has always been clear that as the threat changes, so should defence spending.
"In 2020 the prime minister reversed decades of under-investment in defence and he rightly responded to Russia's danger by continuing to invest in defence, for which the defence secretary is very grateful."
NATO in 'robust health'
Mr Johnson described the NATO alliance as in "united" as the Madrid summit drew to a close.
"The NATO alliance is plainly in robust health and getting stronger, with new members and a new purpose," he said.
"We can see that our work is cut out… If history is any guide, then I believe that this great alliance will again be successful."
The PM also vowed to continue to intensify military support for Kyiv and support the country "no matter how difficult it may be".
"It is only logical and right to continue to support Ukraine in the way that we are and, indeed, to continue to intensify that military support," he continued.
Meanwhile, also speaking at the summit, US President Joe Biden said that NATO is more secure now than before the Russian invasion began.
When asked how long drivers should pay higher prices for gas during Russian war with Ukraine, President Biden replied: "As long as it takes."
Zelenskyy praises UK for additional military support
Earlier on Thursday, the UK government announced it is to provide an extra £1bn of military support to Ukraine. It brings the total UK military support since the outbreak of war to £2.3bn - more than any country other than the US.
The prime minister said the additional money will go towards capabilities including sophisticated air defence systems, uncrewed aerial vehicles, innovative new electronic warfare equipment and thousands of pieces of vital kit for Ukrainian soldiers.
Posting on social media, Ukrainian President Volodymyr Zelenskyy thanked the UK for the additional funds.
Speaking to Sky News this morning, Liz Truss warned that NATO members must make it their "absolute priority" to stop the Ukraine war and push Russian troops out of the country - or face a "much less safe Europe".
Her comments came as the Ukrainian defence minister, Oleksiy Reznikov, told Sky News the Third World War "has already started".