Donald Trump faced calls from Labour to set out his plan to increase American military spending to 5%.
The President-Elect shocked Nato allies by calling for a huge increase in defence spending to ward off the threat from Russia.
But as Labour struggles to meet demands for to reach 2.5% of GDP, David Lammy called on the Mr Trump to explain how he would hit a target of double that.
The Foreign Secretary also said it is unlikely the incoming President will be able to secure a peace deal in the Ukraine war when he comes to power, adding “we have heard the timetable has moved to Easter”.
He said of Mr Trump’s call to increase defence spending to 5%: “He’s right to emphasise that we have to move, and certainly move beyond 2% now.
“I heard the 5%. The United States is at 3.38% of GDP, so I assume that he would set out a roadmap to get to that 5%.”
Insisting European security was on a knife-edge, Mr Lammy said UK intelligence had not “seen any sense at all that Putin is ready seriously to negotiate”.
He added: “I am absolutely clear Donald Trump is an individual who wants a deal, but he is also an individual who sees himself as a winner and not a loser.”
Mr Lammy said: “There had been a slight pushback in Washington that a deal can be achieved on January 21. That is now unlikely and we have heard the timetable has moved to Easter.” Mr Trump has sensationally refused to rule out taking military action in Greenland.
But the Foreign Secretary said: “I think that experience demonstrates that Donald Trump’s intensity of rhetoric and his desire to pursue foreign policy through a lens of ‘peace through strength’ and through a degree of unpredictability, is one of his signatures and that’s the environment we are in.
“There are advantages to that, I think, when you’re dealing particularly with the world’s autocrats, but that is the way he approaches it.”
Mr Lammy said there was sometimes a gap between the rhetoric and the actuality, pointing out that although in his first term he was “pretty robust about Nato” he “actually increased the amount of US troops stationed in Europe, he sent the first Javelins (anti-tank missiles) to Ukraine.”
Asked about Financial Times reports that Mr Musk, the X social media site owner and a member of Mr Trump’s inner circle, has held a private discussion about removing the Prime Minister from office, Mr Lammy said: “My own view is you can’t have free speech if it’s not based on fact and on truth.
“And I’ve had real concerns about some of what I’ve seen online, particularly.”
Mr Musk has been waging an online campaign against the Prime Minister over the issue of grooming gangs.
Mr Lammy used a speech at the Foreign Office to set out his “progressive realism” approach to global affairs “taking the world as it is, not as we wish it to be”.
He said the post-Cold War era of peace is “well and truly over”, with Europe’s future security in jeopardy.
“This is a changed strategic environment. The number of conflicts is higher than at any time since 1945, a spectre of famine from Gaza to Sudan and the most refugees and displaced people on record.”
The Foreign Secretary said that he is sometimes asked when “will things get back to normal,” and added: “My answer is that they will not.
“Europe’s future security is on a knife edge.”
Against that backdrop, Mr Trump and his incoming vice president JD Vance were “simply right when they say that Europe needs to do more to defend its own continent”.
“It’s myopia to pretend otherwise with Russia on the march. So this Government will lay out a clear pathway to reaching 2.5% of our GDP on defence,” Mr Lammy said.
The Nato commitment is for members to spend 2% of gross domestic product on defence, but Mr Trump has suggested that should rise to 5% – a figure far beyond the current US spending of around 3.4%.
The Foreign Secretary said: “I think when Donald Trump came to office (in 2017), only six members of Nato were up to 2%. On leaving office. I think it was around about 18. It’s now at 23 but there are laggards and with war in Europe, he’s right to emphasise that we have to move and certainly meet 2%.
“Now, I heard the 5% the United States is at 3.38% of GDP. So I assume that he would set out a road map to get to that 5%. We have been very clear that we will set out that pathway to 2.5%.”
In his speech, Mr Lammy also set out his hopes that using artificial intelligence could free up diplomats from desks in the Foreign Office to go out to global posts.
He suggested AI could be used to draw up briefing notes for ministers.