The US has demanded Britain hikes its defence spending in a leaked letter from the head of the Pentagon.

In a warning shot to Theresa May, Jim Mattis said he is "concerned" Britain's power could be "eroded" and pointed out France was ploughing cash into the armed forces.

It is the latest in a string of leaks from the Ministry of Defence, whose top minister, Tory Gavin Williamson, has been accused of trying to launch a leadership bid.

By coincidence, Mr Williamson has also been pushing for more cash to be spent on defence.

US defence secretary General Mattis wrote to Mr Williamson ahead of next week's Nato summit - and a visit by Donald Trump to Britain.

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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.

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 letter, leaked to the Sun, was sent on June 12 and will reignite a Whitehall battle over defence funding.

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."

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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."

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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.

Downing Street pointed out that the UK has "the biggest defence budget in Europe" and would continue to meet the 2% target.

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Challenged on whether the UK would continue to have the biggest defence budget in Europe, the Prime Minister's official spokesman said: "In terms of ongoing funding commitments I can only speak for the UK, but you can see what they are."

Asked if it was important to remain the US's main partner, the spokesman said: "We, the US and France are close allies and partners who are willing to act together in defence of our shared values and interests."