“Plucking a date” by which to be spending 2.5% of national income on defence is “the politics of yesterday”, Sir Keir Starmer has said as he defended first conducting a strategic defence review.

Decisions on reaching the 2.5% target will follow a major assessment being launched next week and must comply with the Government’s strict fiscal rules on spending and borrowing.

The Prime Minister, who is attending the Nato summit in the US, said carrying out the review underlined his “ironclad” commitment to 2.5%.

Labour has said it will aim to complete the review in less than a year, but has faced pressure from opposition critics and former military chiefs who say 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.

Security minister Dan Jarvis (Danny Lawson/PA)

“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 gross domestic product (GDP) within his first term in office, telling Sky News: “I’m not going to put a date on it.”

Earlier, security minister Dan Jarvis said the Government is “not in the business of cutting corners” when pressed on the timeline.

He told Sky News: “It’s important that we follow the process and look carefully at the nature of the threat that our country faces, and the resources that will be required to put in place an armed forces that is fit for purpose… it’s an important process that will take a period of months.”

Mr Jarvis also defended the Prime Minister against accusations of hypocrisy after he urged Nato countries to honour their official goal of spending 2% of GDP on defence while the UK reviews its own funding.

Mr Jarvis said that characterisation “would be unfair”.

Asked about his demand of European Nato nations, Sir Keir said their 2% target and the UK’s commitment to achieving 2.5% are “two different things”.

The UK already meets the alliance’s goal, he said, “so we want all allies to make that contribution”.

The Nato summit comes as the alliance debates how to “Trump-proof” its future and its support for Ukraine in case former US president Donald Trump is returned to the White House in November’s election.

The Republican candidate has been a vocal critic of European nations not doing enough to fund their own defence and has threatened to withdraw support for Ukraine.

Twenty-three Nato members are thought to have reached the goal of spending 2% of GDP on defence.

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 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.”