Keir Starmer will give a press conference this afternoon - a day after a chilling Government dossier said Brits must "actively prepare" for war.
The PM will have crunch meetings at the NATO summit in The Hague, with US President among the world leaders who have jetted in. It comes 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.
READ MORE: 13 terrifying warnings from Government report telling Brits to 'actively prepare' for war
"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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