Keir Starmer has said Europe has made a "fundamental shift in its posture" by ramping up security spending.
He said NATO member states have agreed to raise spending to 5% within a decade, reaching 4.1% in 2027. It comes a day after a chilling Government dossier said Brits must "actively prepare" for war.
The Prime Minister said he will oversee the "biggest strengthening" of the UK's nuclear deterrence in three decades after confirming RAF pilots will get nuke-capable jets. He added that NATO is sending a "decisive message to aggressors".
The PM is at The Hague in the Netherlands for a crucial summit amid growing alarm over the spread of nuclear weapons. US President Donald Trump jetted in for the crunch talks, which will see member states pushed to ramp security spending up to 5% of GDP.
Mr Starmer said Mr Trump is a reliable ally to the UK., claiming the relationship between the two nations is "as strong now as it's ever been". There have been questions about Trump's commitment to defending allies, after he told reporters yesterday that his commitment to Article 5 of the bloc's pact - that an attack on one is an attack on all - depends "on your definition".
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The PM was seen in deep conversation with the US President. Mr Starmer's remarks today come a day after a National Security Strategy said the years ahead will test the nation - with a World War Two-style spirit needed to deal with growing threats.
It said tackling the danger of nuclear weapons will be "more complex than it was even in the Cold War". The document went on to warn that major powers like China and Russia are seeking to gain an upper hand in "outer space, cyberspace, the deep sea, and at the Arctic and Antarctic poles".
The document says: "The years ahead will test the United Kingdom... The direction it takes – and the decisions we take – will reverberate through the decades.
"We will need agility and courage to succeed, but we should be optimistic. We remain a resolute country, rich in history, values and in our capabilities. But most of all, there is the determination of the British people themselves. After all, we do not need to look too far into our history for an example of a whole-of-society effort, motivated by a collective will to keep each other safe.

"We can mobilise that spirit again and use it both for our national security and the rebuilding of our country."
Defence Secretary John Healey said the PM trusts that Trump's America would come to the aid of NATO allies. He told Times Radio: "Do I trust President Trump and the US's commitment to Article 5? Yes.
"So does our Prime Minister, and he does because in the Oval Office on his visit to the White House, President Trump gave him that commitment in public."
Mr Starmer has called on Israel and Iran to maintain the pause in hostilities. In a conversation with the French and German leaders at on Tuesday, he "reflected on the volatile situation in the Middle East," according to a Downing Street spokeswoman.
The leaders agreed that "now was the time for diplomacy and for Iran to come to the negotiating table", the spokeswoman added. It comes as intelligence reports in the US suggested that the American attack on Iran's nuclear programme over the weekend have only set it back by a few months, rather than destroyed it as Donald Trump previously suggested.
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