Keir Starmer has admitted he needs to “turn things around” after his defence secretary, John Healey, resigned in a blazing row over military spending that has left Whitehall pointing fingers and the US raising an eyebrow. Downing Street and the Treasury spent Friday trading blows with Healey's allies, with No 10 expressing shock that the Ministry of Defence wanted £18bn to plug funding gaps in major projects - while those close to the former defence secretary accused Starmer of failing to notice that the world has become a slightly more dangerous place.

The prime minister, speaking to the BBC, promised to fight any leadership challenge from Andy Burnham and fended off accusations that he had underestimated the need to spend more on defence. Starmer, however, said anybody who replaced him would face the same set of difficult tradeoffs, justifying his decision to restrict funding on the defence investment plan at a time when his premiership is in jeopardy. “I would just gently say this: that whoever is prime minister is going to face the same prevailing winds as I am facing. None of that is going to change,” Starmer said.

No 10 sided with the Treasury on Friday to complain that it had been surprised by a Ministry of Defence demand for more cash after the strategic defence review last year. But allies of Healey hit back, noting that the geopolitical environment had worsened following the US-Israeli attack on Iran, while Starmer had committed the UK to potential peacekeeping missions in Ukraine and the strait of Hormuz. “Anyone with a brain can see that the world has changed in the past year,” an ally of Healey said, rather pointedly.

Healey quit on Thursday, accusing Starmer of putting the country’s security at risk and being unable to stand up to his chancellor, Rachel Reeves. No 10 and the Treasury offered to plug £13.5bn of an £18bn deficit in spending on defence projects. Healey also revealed that Starmer had only offered to increase defence spending to 2.68% of GDP by 2030, about £25bn below the Nato target of 3.5% by 2035 - a target the prime minister himself agreed to a year ago.

Government sources said there was unhappiness that the MoD had told ministers about a major shortfall during an already agreed spending review, wanting commitments in advance after 2029. The Treasury was equally baffled. “The question is, how did we get from being told the defence review was fully funded to being told there was a £18bn black hole? We still don’t have the answer to that,” a Treasury source said.

There were also signs of concern from across the pond. Elbridge Colby, the influential US undersecretary of defence, cited Healey’s resignation letter and appeared to call for the UK to spend more. “There is again a great need for more British military strength in this critical time,” he said in a social media post. The New York Times reported that the US also wanted to reduce its military presence in Europe, cutting F-15 and F-16E fighter jets from 150 to 100, as it expects the UK and other European Nato members to take the lead on defending against Russia.

Next week, Starmer will face Donald Trump and other G7 leaders at a summit in France, where he will have to deal with the fallout. There was no immediate sign of a turnaround on defence spending. Starmer met Healey’s replacement, Dan Jarvis, on Friday morning and Richard Knighton, the chief of the defence staff, to discuss the budget crisis. The planned publication of the defence investment plan on Monday has been delayed until close to the Nato summit in Ankara, Turkey, at the beginning of July.

“Jarvis inherits what the MoD considers an inadequate settlement,” said Matthew Savill, a defence policy specialist with the Royal United Services Institute thinktank. Jarvis spent his first day in the job visiting a drone testing site in Swindon, which Starmer had been due to visit until Healey resigned. The planned public event took place behind closed doors, with employees told not to mention the defence investment plan or the crisis of the past 24 hours.

Al Carns, the armed forces minister, also resigned late on Thursday, accusing the government of not spending enough on the military and spending on the wrong weapons. With Burnham increasingly expected to win next Thursday’s Makerfield byelection and return to Westminster, many Labour MPs see a leadership challenge as imminent. It could also involve the former health secretary Wes Streeting, who quit Starmer’s government last month.

Asked if he would lead Labour into the next general election, Starmer replied: “Well, that’s what I want to do. I recognise that, you know, I’ve got to turn things around. We had a very bad set of elections.” Asked if he recognised he was in peril, the prime minister replied: “Look, I recognise that, given where we are, I need to turn that around and that’s what I intend to do.”