Nato chiefs have delivered a stark warning to Keir Starmer over the consequences of failing to boost defence spending.
The PM has been told that the UK faces falling towards the bottom of the alliance's league table without more investment.
Top officials are said to have highlighted the danger of failing to hike budgets as a proportion of GDP, with Britain missing targets for bolstering capabilities.
The intervention emerged as Defence Secretary John Healey snubbed a call from Boris Johnson for troops to be deployed to Ukraine immediately.
The ex-PM suggested that putting 'boots on the ground' to keep the peace could 'flip a switch' in Vladimir Putin's head, convincing him Europe is serious about supporting Kyiv.
Mr Healey said he hoped to send forces to Ukraine this year, but suggested that would only be if a peace deal has been negotiated.
Sir Keir has admitted he 'must go faster' on defence spending amid the Russian threat and pressure from Donald Trump.
Keir Starmer has been told that the UK faces falling towards the bottom of the alliance's league table without more investment
The PM has committed to expenditure going from 2.4 per cent of GDP to 2.5 per cent by 2027, with an 'ambition' of reaching 3 per cent in the 2030s
The aftermath of a Russian strike on Kyiv today, as the war ticks over into a fifth year
Defence Secretary John Healey has snubbed a call from Boris Johnson (pictured giving a BBC interview with former UK forces chief Tony Radakin) for troops to be deployed to Ukraine immediately
The PM has committed to expenditure going from 2.4 per cent of GDP to 2.5 per cent by 2027, with an 'ambition' of reaching 3 per cent in the 2030s.
Downing Street aides are now said to want to meet the higher goal much earlier - before the next election.
However, meeting the costs - estimated at £14billion a year - would be a huge challenge after Labour MPs blocked efforts to trim the spiralling benefits bill.
The premier is also under pressure to splurge more on public sector services and wages after barely surviving a coup bid last week.
According to the Sunday Times, the UK's permanent representative to Nato, Angus Lapsley, and assistant secretary general Nick Catsaras have both flagged concerns about spending levels.
Royal Navy admiral Keith Blount, Nato's outgoing deputy supreme allied commander Europe, is said to have voiced similar views in private.
The UK has been trying to make the argument that more weight should be put on the cost of maintaining the nuclear deterrent - not borne by other countries such as Germany, which is ramping up spending.
Mr Catsaras has told the government the UK is 31st out of 32 member states for progress on meeting 2025 capability targets.
Nato spokesman Colonel Martin O'Donnell told the newspaper: 'I am not going to elaborate publicly on any private conversations that Nato leadership have had with anyone.
'What I can say is that Nato has made it crystal clear: European allies and Canada must do more. Delivering a credible defence spending path to 5 per cent as agreed by all 32 nations in the Hague as well as real military capabilities to the alliance is a necessity now more than ever, and a long-term investment in Euro-Atlantic security.'
Writing in The Telegraph ahead of the fourth anniversary of Russia's invasion, Mr Healey said: 'I want to be the Defence Secretary who deploys British troops to Ukraine – because this will mean that this war is finally over.
'It will mean we have negotiated peace in Ukraine. And a secure Europe needs a strong, sovereign Ukraine.'
Britain and France have spearheaded discussions with allies on a 'coalition of the willing' that would deploy peacekeeping forces if a deal is reached to end the war.
Mr Healey said there was 'no heavier burden on any Defence Secretary or any government than committing our Armed Forces on operations'.
Mr Healey said he hoped to send forces to Ukraine this year, but suggested that would only be if a peace deal has been negotiated
Mr Johnson warned that Putin would likely 'keep going' until he was convinced of the West's determination for the war-torn country to be free and independent.
Sitting alongside former head of the military Admiral Sir Tony Radakin, Mr Johnson told the BBC's Sunday With Laura Kuenssberg show there was 'no logical reason' why there should not be non-fighting boots on the ground in Ukraine in the immediate future.
'If we can have a plan for boots on the ground after the war, after Putin has condescended to have a ceasefire, then why not do it now?' he said.
