Sir Keir Starmer has said it is “essential” for Nato allies to increase defence spending because of the “grave threats” to security.
The Prime Minister has committed to spending 2.5% of gross domestic product on defence but he has not set a timetable to reach that goal.
Nato members are meant to spend at least 2% on defence and the potential for Donald Trump’s return to the White House after November’s election means there is an urgency for states to meet that goal or go even further.
Mr Trump has been publicly critical of European nations depending on the protection of the US without spending their own cash on the military.
As the Nato summit drew to a close in Washington, Sir Keir said: “Today, this alliance is stronger than ever.
“Twenty-three members are now spending 2% of their GDP on defence.
“But in light of the grave threats to our security, we must go further.
“So we will conduct a strategic defence review to strengthen our armed forces and protect our national security.
“And we will set out a clear path to spending 2.5% of our GDP on defence.
“I say with candour to all our allies, we must understand that this is now essential.
“This is a defensive alliance. We do not seek conflict.
“But we know that the best way to avoid it is to prepare for it and to lead.”
The Prime Minister was repeatedly challenged about his refusal to put a timeframe on reaching 2.5% of GDP during a round of broadcast interviews.
Opposition critics and former military chiefs have warned the decision cannot wait given the gravity of threats facing Britain.
Before his election defeat, Rishi Sunak had committed to reach 2.5% by 2030 at a cost of £75 billion over six years.
But Sir Keir hit out at this “arbitrary” deadline.
He told the BBC in Washington DC on Thursday: “We will go through the strategic review. I think that’s very important because there’s information which we weren’t privy to when we were in opposition, which we are now privy to. We want to go through that exercise.
“The commitment is absolutely ironclad to that 2.5%. I think that being clear that it’s within our fiscal rules, and therefore there has to be a road map, actually represents our big commitment to it.
“What I don’t want to do is what has been done in the past, which is to set an arbitrary date without a pathway. I don’t think that’s serious.
“We’re so serious about it that we want to make sure that it is within our fiscal rules, that we can show exactly how it will be delivered.
“But it will only be delivered when we have that pathway. To be plucking a date is the politics of yesterday.
“I am determined that we are seriously going to put a plan in place to reach that commitment, which I made very, very clearly.”
Sir Keir again refused to guarantee he will meet the goal of spending 2.5% of GDP within his first term in office, telling Sky News: “I’m not going to put a date on it.”
The Tories accused the Labour Government of creating “huge uncertainty” by declining to match their pledge to reach 2.5% by 2030.
Shadow defence secretary James Cartlidge said: “This morning, the security minister (Dan Jarvis) was forced to defend Keir Starmer for calling on Nato members to increase defence spending when the Labour Government have not set a date to reach 2.5% of defence spending by.
“In a world that is more volatile and dangerous than at any time since the Cold War, Keir Starmer’s Labour Government had a clear choice to match the Conservatives’ fully funded pledge to spend 2.5% of GDP on defence by 2030. By failing to do so, they’ve created huge uncertainty for our armed forces, at the worst possible time.”
