John Healey, Britain’s defense secretary, resigned on Thursday, accusing Prime Minister Keir Starmer of failing to invest enough money in its military and leaving the country “well short” of what it needed to protect itself in a dangerous world.
Mr. Healey, who has been one of the prime minister’s staunch allies, said he was informed on Monday that Mr. Starmer intended to announce a plan that would not increase military funding as much as Mr. Healey believes is necessary.
“I am being forced to make decisions that would reduce the readiness of our forces and increase the risk to personnel on operations, and could make the country less safe,” Mr. Healey wrote in a letter to the prime minister that he posted on social media.
The unexpected resignation is a significant blow to Mr. Starmer, who is likely to face a challenge to his leadership of the Labour Party later this summer. Mr. Healey’s clear lack of confidence in the prime minister bolsters the case of his critics, who insist the party needs a change at the top.
Mr. Healey said that Mr. Starmer’s Defense Investment Plan would increase military spending to 2.68 percent of gross domestic product by 2030, short of the 3.0 percent that Mr. Healey and Mr. Starmer had discussed in the past.
“After explaining to you that I would not be able to accept a DIP settlement that does not give our forces the resources they need,” Mr. Healey wrote, “I am now left with no other option than to submit my resignation as your defense secretary.”
Questions about how much Britain plans to invest in the military have swirled through the halls of government for months as Mr. Starmer repeatedly delayed releasing the long-term defense plan.
A year ago, Mr. Starmer’s government published a strategic defense review, which called for a significant improvement to military readiness, including the addition of British-built long-range weapons, the creation of a cyber command, and the development of drones and new ways to protect undersea cables.
In a speech at a British defense company last week, the prime minister warned of the dangers that Britain and Europe face in the coming years.
“It is our intelligence assessment, and the assessment of other countries in NATO, that there could be an attack by Russia on NATO as soon as 2030,” he said.
Mr. Healey took note of those comments in his resignation later, saying they were evidence that “you know what defense needs.”
There has also been immense pressure from President Trump, who has repeatedly accused Britain and other nations in Europe of failing to spend enough on their own defense. Mr. Trump has mocked the British Navy as being “too old,” with aircraft carriers “that didn’t work.”
But the sluggish economy in Britain has made it challenging for Mr. Starmer to find the money for a big lift in military spending without making politically unpopular decisions such as tax increases, cuts in other domestic programs or large-scale borrowing.
Each of those options comes with steep downsides. Tax increases would be highly contentious in a country where levies on the population are already relatively high. Many people in Britain remember with anger the steep cuts to popular services, known as austerity, that were put in place by previous Conservative Party-led governments. And financial markets would likely respond poorly to more government borrowing.
Mr. Starmer has come under pressure from the left flank of his party to invest more in the National Health Service, infrastructure, transport and other domestic programs that suffered years of capital underinvestment before Labour took power. That has been part of the argument made by Andy Burnham, the mayor of Greater Manchester, who is running for a seat in Parliament so he can try to replace Mr. Starmer as prime minister.
The prime minister’s office issued a statement Thursday saying that “the country is safer because of the decisions Keir Starmer has made and we will continue to act in our national interest.” The statement added: “The Defense Investment Plan will deliver the capability our armed forces need.”
In his letter, Mr. Healey appeared to lay much of the blame for lower military spending on Rachel Reeves, Britain’s top finance official, who oversees the Treasury. He told Mr. Starmer that he had initially been hopeful that there was agreement on the need to boost the country’s military.
“Since then, you have been unable, and the Treasury has been unwilling, to commit the resources that the nation needs to defend the country at this time of rising threats,” he wrote.
Ms. Reeves said this week that tax increases would be better than borrowing to pay for defense. Aides to Mr. Starmer have declined to say whether he agreed with her.
Government officials refused to say as recently as Wednesday afternoon what Mr. Starmer had decided to do. A spokesman told reporters on Wednesday that the investment plan would be unveiled before a NATO meeting at the beginning of July.
In Parliament on Wednesday, Kemi Badenoch, leader of the Conservative Party, accused Mr. Starmer of delaying because of concerns about how to pay for the defense investments.
“Projects have been put on hold, and Britain is getting weaker with every passing day,” she said. “At a time of increased global instability and tension, with war in Europe and the Middle East, the prime minister is paralyzed, giving the armed forces less than half of the minimum that they need.”
Mr. Starmer responded by noting that military spending had fallen during the 14 years that Ms. Badenoch’s party was in power. He noted that his government had already agreed to increase it to 2.6 percent, from 2.3 percent, by 2027.
“We have carried out a strategic review of defense, and we are committed to publishing the defense investment plan before the NATO summit,” he said.
Tony Travers, a professor of politics at the London School of Economics, said that Mr. Healey’s resignation carried weight in Britain because he was not regarded as a ruthlessly ambitious politician or someone prone to rash decisions.
A lifelong Labour politician, Mr. Healey has served in Parliament for decades, serving every Labour prime minister since Tony Blair, starting in 2001.
“He’s a sort of rational moderate. Almost the ultimate exemplar of a rational politician,” Mr. Travers said. “He’s definitely not somebody who is prone to sudden and unexpected career moves, and so this suggests he was very, very disgruntled.”
