The US has stepped up pressure on the UK to increase its military spending ahead of a crunch meeting between Donald Trump and fellow Nato leaders including Theresa May.
Pentagon chief Jim Mattis wrote to Defence Secretary Gavin Williamson to hint that the special relationship could be under strain unless the UK boosted its defence capability.
The US defence secretary told his British counterpart that Washington is "concerned" that the UK's military power and diplomatic influence is "at risk of erosion".
General Mattis said he wanted the UK to remain America's "partner of choice" but also said that the French were committed to being "global actors" alongside the US.
The letter, leaked to the Sun, was sent on June 12 and will reignite a Whitehall battle over defence funding.
Mr Williamson has been pushing for extra cash, stepping up his efforts after the NHS was promised a funding boost.
US president Mr Trump will travel to Europe next week for a potentially stormy Nato summit on July 11-12, before heading to the UK.
In the two-page letter, Gen Mattis said "the credibility of the UK's armed forces" had been crucial in securing its place on the world stage.
But he added: "I am concerned that your ability to continue to provide this critical military foundation for diplomatic success is at risk of erosion, while together we face a world awash with change."
The UK is committed to meeting the Nato target of spending 2% of GDP on defence, but the US hinted that it wants Mrs May to commit more money.
Gen Mattis said: "A global nation like the UK, with interests and commitments around the world, will require a level of defence spending beyond what we would expect from allies with only regional interests.
"Absent a vibrant military arm, world peace and stability would be at further risk."
Highlighting Emmanuel Macron's recent pledge to pump an extra £260 billion into defence by 2025, Gen Mattis said: "As global actors, France and the US have concluded that now is the time to significantly increase our investment in defence.
"Other allies are following suit."
He stressed that it was "in the best interest of both our nations for the UK to remain the US partner of choice" but "in that spirit, the UK will need to invest and maintain robust military capability".
In a reference to the Whitehall rows over funding, Gen Mattis added: "It is not for me to tell you how to prioritise your domestic spending priorities, but I hope the UK will soon be able to share with us a clear, and fully funded, forward defence blueprint that will allow me to plan our own future engagement with you from a position of strength and confidence."
A Ministry of Defence spokesman said: "The UK maintains the biggest defence budget in Europe and we have been clear we will continue to exceed Nato's 2% spending target.
"The Defence Secretary launched the Modernising Defence Programme to strengthen our Armed Forces in the face of intensifying threats."
The week after receiving the letter, Mr Williamson told MPs that US patience over European defence spending was wearing thin.
"We can't continue to expect US taxpayers to keep picking up the tab for European defence indefinitely, nor can we expect US patience to last forever," he said on June 20.
The Commons Defence Committee has also called for a significant funding boost, moving the level of defence spending up from 2% to 3% of total GDP.
A cash injection on this scale would equate to additional funding of around £20 billion a year and bring investment in defence to levels similar to those seen between the end of the Cold War and the mid-1990s.