Keir Starmer will face his first international test just days after becoming prime minister as he meets President Biden and other world leaders at the Nato summit in Washington.
The Labour leader will try to reassure the nuclear alliance of his party’s “unshakeable” support under his leadership.
The meeting comes two years after his predecessor Jeremy Corbyn said he hoped organisations like Nato could be ultimately disbanded and said they could create “greater danger” in the world.
The new prime minster will also restate the UK’s commitment to Ukraine at a time when questions are being asked about US support, amid turmoil over the looming presidential election.
Sir Keir’s first face-to-face meeting with President Biden risks being overshadowed by calls for the 81-year-old to pull out of the race for the White House.
A disastrous showing in a televised election debate 10 days ago has left senior Democrats fearful their candidate risks losing disastrously to Donald Trump.
The possible return of the former president, who has openly questioned continued backing for both Ukraine and Nato, is set to cast a shadow over the meeting.
Nato leaders are also facing political uncertainty in Europe, where the decision of President Macron to call a snap election appears to have backfired for the French leader.
Russia’s attack on a children’s hospital in Ukraine is set to add extra urgency to the organisation’s discussions on the war there.
The summit, designed to celebrate the 75th anniversary of Nato’s founding, will be the first since Sweden joined the alliance after Russia’s invasion of Ukraine.
Labour’s defence secretary John Healy has already visited the war-torn country, in a move designed to emphasis the UK’s backing is for the long haul.
Sir Keir also spoke to Volodymyr Zelensky on his first day in office and with the Ukrainian leader also set to be in Washington the two leaders could hold talks.
It would not be the first time the two men have met. They chatted during last month’s D-Day event after the then prime minister Rishi Sunak infamously left early to return home for an interview with ITV.
Sir Keir also spoke to President Biden on his first day in the job. The American leader told him there was “no doubt” the special relationship between the UK and the US would continue as he congratulated him on a “hell of a victory”.
He added: “There’s no doubt under your leadership our two countries are going to continue the special relationship. We’re working together on just about every issue, supporting Ukraine, managing the competition with China, advancing cooperation in the Indo-Pacific with AUKUS.”
In Washington, the PM is expected to be joined by foreign secretary David Lammy as well as the Mr Healey and Nick Thomas-Symonds, the minister for Europe.
In his first press conference as PM on Saturday, Sir Keir said: “It is for me to be absolutely clear that the first duty of my government is security and defence, to make clear our unshakeable support of Nato.”
Labour’s manifesto pledges to spend 2.5 per cent of GDP on defence “as soon as we can”.
On Sunday, Mr Lammy wrote in an article on the Local Europe website : “Next week, the Prime Minister, the Defence Secretary and I will travel to Washington for the Nato summit.
“Seventy-five years ago, my political hero and former Labour foreign secretary, Ernest Bevin, was pivotal to forming the alliance … he would have been delighted to see Nato grow larger over the years with Germany, Poland and now Sweden all joining the most successful defensive alliance in history.
“This Government’s commitment to the alliance is unshakeable, just as it was in Bevin’s time. I will be discussing this weekend how Nato allies can go further in investing in our tightly connected defence industries and providing Ukraine with its own clear path to joining our alliance.”
