Donald Trump's administration is said to hold reservations over Britain's faltering defence spending commitments, setting up an imminent clash with likely prime minister Andy Burnham.
Three individuals, among them a senior administration official, revealed that Washington believes the UK and other European nations have offered a "false promise" in agreeing to the president's demands for increased defence budgets.
The White House are reportedly not confident that Britain will honour a commitment to allocate 5 per cent of GDP to military spending, according to sources who spoke to The Telegraph.
This scepticism from the US threatens an early confrontation with Andy Burnham - widely expected to succeed Sir Keir Starmer as Prime Minister.
The Trump administration's concerns have prompted a reassessment of American military presence across Europe, with officials warning US assets could be withdrawn from the continent if allied nations fail to increase contributions.
Mr Burnham has privately indicated to allies of his intention to boost defence expenditure, but finds himself at odds with Downing Street over Sir Keir's determination to proceed with the long-delayed defence investment plan.
Sir Keir is expected to announce a £15billion uplift in military spending through the Dip this coming week, though the document will omit any specific date for reaching the 3 per cent threshold defence chiefs have requested.
John Healey, who stepped down as defence secretary on June 11, had advocated for a 2030 deadline.
While Britain has achieved the initial 2 per cent target demanded by Mr Trump, further increases have proved challenging, with the UK's proportional defence spending now ranking below that of 13 fellow Nato members.
Mr Trump is anticipated to travel to Ankara on July 7 for the annual Nato summit, though fellow leaders remain anxious he might withdraw the United States from the alliance entirely.
The President's confidence in Mark Rutte, Nato secretary general, also appears to be waning.
"There's a big frustration growing that Mr Rutte is just a talker. He hasn't been able to deliver," a senior US administration official said.
American personnel stationed at Nato's Brussels headquarters are reportedly "nervous" that Mr Trump may skip the Ankara event altogether.
Relations have deteriorated since the Iran conflict began on February 28, particularly after Britain refused to permit US forces to launch strikes from the Diego Garcia base in the Chagos Islands - leaving the UK seen as an unreliable partner.
A White House spokesman said alliance members must assume greater "responsibility for their own defence" when questioned about Britain's spending plans.
The spokesman said: "The United States has done more for Nato than anyone else. President Trump has made his disappointment with Nato clear, and the president remains firm.
"Nato members must take greater responsibility for their own defence and all of them should have been there for the United States when we were in the middle of the successful Operation Epic Fury."
Mr Rutte's anticipated progress report at the summit is expected to show European allies and Canada have collectively raised their defence budgets by 20 per cent, equivalent to $100billion.
A senior European diplomat told the Telegraph: "We've heard American scepticism loud and clear. They will typically say, 'If all European countries were like the Nordics, we would be in a much better position, but will some of the bigger European countries ever deliver?' They doubt that."
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