Sir Keir Starmer will tell NATO allies that the UK has a “cast iron commitment” to lifting the UK’s defence spending to 2.5% of GDP, but is refusing to honour this pledge in his first term in government.

The new Labour prime minister said en route to his first international summit since his landslide election win last week that the UK “must make sure we are ready to defend ourselves” as he ordered an immediate “root-and-branch” review of Britain’s defence capabilities.

Sir Keir will launch the strategic defence review next week, which will put a ‘NATO-first’ approach at the heart of Britain’s defence plans.

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But the Labour leader also refused to commit to hitting the 2.5% target in his first term, saying only he would like to do it “as soon as possible” as he urged other allies to step up spending to “safeguard the future of the Alliance”.

He is expected to use NATO’s 75th-anniversary summit in Washington to argue the case for all allies to adopt 2.5% as the new target, as opposed to the current target of 2% of GDP.

Asked if he would set out a timeline to hit the target, the prime minister said: “The most important thing at the moment, at this NATO summit, is to recommit to and stand in solidarity with Ukraine and discuss the practical plans for further action that we can take together in relation to Russian aggression. That is really important.

“I’m committed to that 2.5% within our fiscal rules. That strategic review needs to come first, but today and tomorrow and the next day is all about standing together with our allies, discussing practically how we provide further support for Ukraine and send a very, very clear message to Putin that we will stand against Russian aggression wherever it is in the world.”

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Sir Keir’s position stands in contrast to that of his predecessor Rishi Sunak, who made it a manifesto pledge to hit the 2.5% target by the end of this parliament.

The UK’s strategic review will determine the future defence posture of the nation, the capabilities needed and will set out a roadmap to achieving 2.5% of GDP on defence.

The government said the review will consider the threats the UK faces, from Russia to the Middle East, and the rise of authoritarian regimes.

One of Starmer’s first international calls as prime minister was to President Zelenskyy, in which the Labour leader reiterated the UK’s unwavering support for Ukraine.

“The prime minister firmly believes that Ukraine’s security is our security,” his official spokesperson said.

The 75th anniversary NATO summit comes as Russia intensifies attacks on Kyiv.

Russian missiles blasted cities across Ukraine on Monday, damaging the country’s largest children’s hospital and other buildings in a fierce assault that interrupted heart surgeries and forced young cancer patients to take their treatments outdoors. At least 41 people were killed in the strikes, officials said.

The summit is also likely to be overshadowed by questions over the viability of President Biden’s re-election bid, as the US president fights off pressure from his own party to stand down.

The US president made opening remarks at the summit on Tuesday. He is expected to hold an unscripted press conference later this week which will be closely watched.

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President Biden this week issued a defiant letter to wavering congressional Democrats telling them that “any weakening of resolve… only helps Trump and hurts us”.

The president also went on national TV to insist “I’m not going anywhere”.

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