AMERICA has given Britain an ultimatum to boost its defence spending, or risk the death of the Special Relationship.

The bombshell warning was delivered to the US’s Secretary of Defense to his British counterpart, Gavin Williamson, two weeks ago.

7 Britain's military is 'at risk of erosion', warns the US Credit: PA:Press Association

The letter - which is unprecedented in its bluntness - has been leaked to The Sun.

In it, former US Marine Corps general Jim Mattis says Washington DC is “concerned” that Britain’s military prowess is “at risk of erosion”.

With threats around the world spiralling, America now expects the UK to hike its cash to the military to help counter them “beyond what we would expect from allies with only regional interests”.

In an ominous threat, the general also makes it very clear that while Britain may want to “remain the US partner of choice”, France is already boosting its defence spending - and is waiting in the wings to replace us.

Theresa May sparks fury by refusing to confirm if Britain will remain a Tier One defence power

7 Jim Mattis sent his warning in a letter to Defence Secretary Gavin Williamson Credit: EPA

President Trump wants an answer from the UK by the time he arrives in Europe next week for the NATO Summit, Mattis adds.

The letter’s revelation today is a deep embarrassment to Theresa May, and plunges her into a fresh political crisis.

Defence Secretary Mr Williamson, backbench MPs and British generals have all pleaded with her to hike the MoD’s budget and fill its £20bn blackhole.

But the PM is adamantly refusing, alongside Chancellor Philip Hammond.

7 Extracts from Mattis' letter

The letter from highly respected General Mattis - nicknamed “Mad Dog” when he was in the Marines - was sent on June 12, three days after he held talks in London with Mr Williamson while his guest at Trooping the Colour.

In the two page letter, General Mattis argues that Britain’s position as a leading world power “has been the credibility of the UK’s armed forces”.

But he adds: “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”.

7 Generals are calling on the PM to increase the MoD’s budget to 2.5% of GDP a year, which would see a cash increase of £6bn-£7bn Credit: PA:Press Association

Britain's new £150m F-35 Lightning fighter jets land in Britain after flight across Atlantic

For the first time in the letter, the senior US politician also gives a firm hint that Washington now wants Britain to go beyond the NATO minimum of spending 2% of GDP on defence, which the government only just about meets at the moment with the inclusion of pensions.

Mattis adds: “A global nation like the UK, with interests and commitments around the world, will require a level of defense (sic) spending beyond what we would expect from allies with only regional interests.

7 More from the letter

“Absent a vibrant military arm, world peace and stability would be at further risk”.

Purposely highlighting Paris’s recent pledge to pump an extra £260bn into bolstering its armed forces by 2025, Mattis adds: “As global actors, France and the US have concluded that now is the time to significantly increase our investment in defense.

“Other allies are following suit.”

And dropping the hint that the Special Relationship is now in peril with France in ther wings, Mattis goes on: “It is in the best interest of both our nations for the UK to remain the US partner of choice.

7 Theresa May has infuriated Tory backbenchers by repeatedly refusing to commit to maintaining the UK’s status as a Tier One military power

“In that spirit, the UK will need to invest and maintain robust military capability.

“It is not for me to tell you how to prioritize your domestic spending priorities, but I hope the UK will soon be able to share with us a clear, and fully funded, forward defense blueprint that will allow me to plan our own future engagement with you from a position of strength and confidence.

“In advance of that, the President and I look forward to hearing details of the progress you have made with your Modernizing Defence Programme at the upcoming NATO Summit.”

Generals are calling on Mrs May to increase the MoD’s budget to 2.5% of GDP a year, which would see a cash increase of £6bn-£7bn.

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In comparison, the US spends 3.6% of the biggest GDP in the world on its military.

But Mrs May has infuriated Tory backbenchers by repeatedly refusing to commit to maintaining the UK’s status as a Tier One military power, saying only that the UK would be a “leading nation” instead.

Former Chief of Defence Staff Lord Houghton warned on Saturday: “US observers have come to realise we are living a lie.

“President Trump is not remotely impressed by our national pretension to be America’s greatest ally.”

The letter will also be seen as more evidence that Mrs May’s relationship with Donald Trump is under increasing strain.

7 Former Chief of Defence Staff Lord Houghton says Britain is 'living a lie' when it comes to its military commitments Credit: PA:Press Association

Fears are mounting in Downing Street that the President will use next week’s NATO summit in Brussels to announce a major US drawdown unless more members states step up their military funding.

The letter – leaked to The Sun by military sources – has been copied to the Chiefs of the three armed forces, as well as No10, and National Security Advisor Sir Mark Sedwill.

Senior Tories last night called on Mrs May to urgently act on Mr Mattis’s warning.

Former defence minister Mark Francois MP told The Sun: “For many months, the Defence Committee has been arguing that we must increase our defence spending, not least to remain a credible ally of the Unietd States.

“This letter is a real wake up call that we need to invest to maintain our leardership in NATO, and I hope the Prime Minsiter takes urgent note.”