The UK requires a "credible path" marked by annual rises to hit its Nato defence spending target, alliance chief Mark Rutte has said.
The Nato secretary general told the Press Association member states "have to stick to that path" towards spending 3.5% of gross domestic product – a measure of economic output – on core defence by 2035, setting out "yearly increments".
This comes as Prime Minister Andy Burnham faces political pressure to outline how Britain will reach that goal.
The Government has so far only budgeted an increase to 2.7% of GDP by 2027/28, with no firm details on how further growth will be achieved beyond that date.
Mr Rutte explained that reaching the 3.5% target cannot happen overnight because Nato nations need time to expand their armed forces and defence industries.
In an exclusive interview with PA on Thursday, the Nato chief said: "You cannot spend it today, but you can also not wait ’til 2035.
"And that is why you need a credible path towards that 3.5%, and yearly increments."
Warning of threats from Russian President Vladimir Putin after a series of hybrid attacks on European soil, Mr Rutte said: "Let’s not be naive about what he might do against Nato."
With Mr Putin willing to “sacrifice 1.2 million of his own people in Ukraine” and spend 50% of his state budget on defence, “we have to make sure that we stay ahead of them”, he said.
“I think we are on the right path now to make sure that also going forward we stay ahead of the Russians.
"But again, that means that all of us, the UK and all the others, have to to stick to that path to the 3.5% with yearly increments."
Chancellor John Healey quit as defence secretary in Sir Keir Starmer’s administration during a row over funding, having pushed for 3% by 2030.
He has since appeared unwilling to recommit to that interim target, saying the timetable for spending 3.5% of GDP by 2035 will be presented at next year’s spending review.
Mr Rutte praised Mr Burnham’s Government for backing the Nato target.
After meeting Mr Burnham on Wednesday, and Ed Miliband and Wes Streeting on Thursday, Mr Rutte told PA: "I’m really happy with the Prime Minister and also all the other key political players, including the Foreign Secretary and Defence Secretary, really committing to the 3.5%, but also committing to the need for a credible path to get there.
"And of course, I understand that next year’s spending review … will probably lead to more decisions on what that path exactly will be."
He stated that allies could not "in 2034 do in one step" the jump to 3.5% from current spending levels, noting there "has to be a glide path".
The head of Nato also appeared to side with the UK in its pushback against the European Union’s efforts to encourage the bloc’s domestic arms industry by limiting the role of outsiders.
Mr Rutte said: “I agree with the British Government that we need to have a defence industrial base which is encompassing the whole of the alliance, from the US up to Turkey.
“And I know some are saying, ‘Hey, buy European or buy this or buy that’, but this type of protection will not help us.”
Following his talks with the UK Cabinet ministers, Mr Rutte said he had pushed to “do more in sharing the big innovation coming out of the UK with the rest of Nato.”
“I was speaking to the converted, but if I have been pressing one thing, it is this: that we need to speed up innovation in Nato, and the UK is a leader here, and with the extra defence spending, it means also being able to spread faster these innovations amongst the Nato alliance.”
