Donald Trump's America waded into the row over funding for Britain's Armed Forces today with a senior official saying the world needed more UK 'military strength'.

US under-secretary of war, Elbridge Colby heaped pressure on Sir Keir Starmer  to act as the beleaguered Prime Minister reeled from the resignation of two of his defence ministers over a lack of cash for the Ministry of Defence.

Sir Keir is under pressure to bring forward a major programme of spending amid fears that the Treasury is trying to provide as little as it can, possibly as little as £10billion, little more than a third of half what experts say is needed.

Ahead of a by-election next week that is likely to trigger a leadership challenge from returning MP Andy Burnham, the PM today effectively begged Labour MPs for more time as he insisted defence is his 'number one priority'.

But sharing the devastating resignation letter from outgoing defence secretary John Healey, Mr Colby highlighted 'the courage, tenacity, dedication, daring, and loyalty' of British servicemen and women in two world wars 'that won them the lasting admiration and gratitude of the whole world'.

'The United Kingdom has an extraordinarily proud military history. It commands our respect,' Mr Colby wrote on X.

'There is again a great need for more British military strength in this critical time. We urge the UK to meet that need with urgency, scale, and determination.'

The Trump administration has long demanded its Nato allies increase their military spending, amid fears that it cannot be relied upon to defend Europe from any Russian attacks.

As Sir Keir's grip on power loosens further:

• The PM faces the prospect of heading to a G7 summit alongside Donald Trump next week with the UK's defence plans in shambles;
• More evidence has emerged of a Russian military build-up on the border with Europe;  
• Mr Carns, a former Royal Marine, has hinted he will stand in any Labour leadership contest;
• The premier still needs to find a replacement for Mr Carns and two ministerial aides, as well as a new security minister; 
• Meg Hillier, a ringleader of a previous Labour revolt against welfare curbs, has said she believes MPs are now ready to support cuts;  

The PM claimed he was being 'bold' on military funding as he reels from the resignation of Mr Healey, who accused him and Rachel Reeves of failing to keep the country safe.

He also vowed to 'fight' a Labour leadership challenge, warning that Andy Burnham or any other successor would face the same 'prevailing winds'.

However, Sir Keir sounded far less definite than before about whether he can hang on, saying he 'wants to' take the party into the next election but knows he needs to 'turn things around'. 

Mr Healey was followed out of the door last night by his deputy Al Carns and two ministerial aides, all condemning the settlement proposed after months of bitter Whitehall wrangling. 

There was an agonising nine-hour wait before security minister Dan Jarvis was finally promoted to the key job - raising questions about whether he is willing to accept the financial proposals. 

This morning Sir Keir staged an awkward Downing Street photo op with Mr Jarvis and Chief of the Defence Staff Richard Knighton - also thought to be at risk of quitting.

In an interview later, the premier insisted: 'I have taken the difficult decisions to make sure that we are safe as a country, because that is my number one priority.'

But he admitted the Defence Investment Plan (DIP) - which was due last Autumn - has still not been finalised. 

And Sir Keir dodged on calls for welfare curbs to help find more money, suggesting that his aim was to get more people into work rather than cut handouts. 

On doubts about his future, Sir Keir said: 'I'm not going to go away. I don't think we should plunge the country into the chaos of a leadership election.' 

He added: 'I don't think it should happen, but if it does then I will fight. And let me just be clear with you. That's not about personal vanity. It's not about stubbornness. It's out of a very deep sense of duty.' 

'The plan is being developed. We are determined to get it right,' Mr Kyle told Times Radio.

'We are talking about an enormous amount of money going into defence at a period of time where we have to modernise the way we think about defence, but also make sure that we do so in a way that benefits British jobs.'

In his resignation letter yesterday, Mr Healey revealed that plans signed off by the PM and Chancellor will increase defence spending by just 0.08 percentage points of GDP by the end of the decade, despite rising threats across the world.

Senior defence figures warned that the threadbare settlement would leave Britain 'in danger'.

The Chief of the Defence Staff, Air Chief Marshal Sir Richard Knighton, is also regarded as on resignation watch.

Mr Carns followed Mr Healey out of the door last night less than an hour after a Sky News interview – not authorised by No10 – in which he said the funding proposals were not good enough.

He has warned that the UK will be seen as weak at a crucial Nato summit starting on July 7. 

Mr Carns declined this morning to pour cold water on claims he is mulling a bid to succeed Sir Keir, saying 'we'll see what happens in the future'.

He also suggested that benefits could be cut to help free up funding. 'We have got to find more money,' he said. 

'There is an argument about welfare. I am a firm believer that it's about hands up, not a hand out. 

'But we need to help the people who need the most help within the nation but also get the balance right on defence.' 

Despite the backlash, Sir Keir has signalled he will stick with the funding plan rejected by Mr Healey.

It is unclear whether the blueprint will appear before he heads to a G7 summit - also attended by Donald Trump - next week.

Sir Keir was already under huge pressure as Andy Burnham threatens to launch a leadership challenge, assuming he wins the Makerfield by-election next Thursday.

Tom Tugendhat, a former soldier who served as a Tory security minister, told BBC Radio 4's Today programme that Mr Healey's words as he resigned were 'damning'.

He added: 'The reality is now the enemy is at the gate, and we're still not taking this seriously.

'So, it's one of those 'If not now, when?' moments.'