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Sir Kim Darroch has resigned as the UK's ambassador to the US after Donald Trump vowed to no longer deal with him over bombshell comments he made in leaked diplomatic memos. 

Sir Kim said he wanted to 'put an end' to the speculation over his future after his heavily critical comments about the Trump administration sparked an all-out diplomatic war between the US and Britain.  

The leaked documents revealed Sir Kim had called Mr Trump 'inept' and described the current White House as 'uniquely dysfunctional'. 

They prompted the US President to launch an unprecedented attack on Sir Kim and Theresa May as he called the former a 'pompous fool' and the latter a 'disaster'. 

Announcing his decision to quit in a letter to Sir Simon McDonald, Permanent Under Secretary at the Foreign Office, Sir Kim said the current furore over his role made it 'impossible' for him to do his job. 

He said: 'Since the leak of official documents from this Embassy there has been a great deal of speculation surrounding my position and the duration of my remaining term as ambassador. I want to put an end to that speculation. The current situation is making it impossible for me to carry out my role as I would like.

'Although my posting is not due to end until the end of this year, I believe in the current circumstances the responsible course is to allow the appointment of a new ambassador.'  

Mrs May said at Prime Minister's questions in the House of Commons this lunchtime that Sir Kim's resignation was a 'matter of great regret' as she appeared to deliver a message to her successor to stand up to Mr Trump. 

She said: 'Sir Kim has given a lifetime of service to the UK and we owe him an enormous debt of gratitude.' 

She added: 'I hope this House will reflect on the importance of defending our values and principles particularly when they are under pressure.'

Her comments came after Boris Johnson last night refused to guarantee that he would keep Sir Kim in post if he replaces Mrs May, after the ambassador had started yesterday to be frozen out of formal events and meetings in Washington. 

Mr Trump called Mrs May 'foolish' and Sir Kim a 'very stupid guy' as he responded to leaked comments from the UK's ambassador in Washington

Sir Kim Darroch labelled Mr Trump 'inept' in a series of diplomatic cables which were leaked. The leak is now the subject of a formal Cabinet Office investigation

Theresa May, pictured leaving Number 10 today, told MPs at lunchtime that Sir Kim Darroch's resignation was a 'matter of great regret'

Whitehall sources told the Financial Times that Sir Kim had made his decision after watching Mr Johnson at the televised Tory leadership debate, having concluded the favourite to win the keys to Number 10 'was willing to throw civil servants under the bus'. 

Jeremy Hunt, the Foreign Secretary and Mr Johnson's leadership rival, had promised not to replace Sir Kim if he became PM. 

Responding to Sir Kim's resignation, Mr Hunt tweeted: 'Standing up for Britain means standing up for the finest diplomats on the world. It should never have come to this.'

The Foreign Secretary said in an official statement 'If one theme ran through all his endeavours, it was his unswerving devotion to upholding the interests of the United Kingdom, in the best tradition of British diplomacy.

'In that spirit, he brought dispassionate insight and directness to his reporting to ministers in London. Whenever I visited Washington as Foreign Secretary, I was struck by Sir Kim's professionalism and intellect. I am outraged that a selection of his reports should have been leaked.

'I am sure that our Ambassadors worldwide will continue to provide the objective and rigorous reporting that the Foreign and Commonwealth Office has always prized.

'I profoundly regret how this episode has led Sir Kim to decide to resign.' 

Jeremy Corbyn, the Labour leader, said Mr Trump's comments about Sir Kim were 'beyond unfair and wrong'. 

'I think he has given honourable and good service and he should be thanked for it,' he said. 

Sir Simon said in a written response to Sir Kim's resignation letter that the outgoing ambassador had been the victim of a 'malicious leak'. 

He said: 'Over the last few difficult days you have behaved as you have always behaved over a long and distinguished career, with dignity, professionalism and class. 

'The Prime Minister, Foreign Secretary and whole of the public service have stood with you: you were the target of a malicious leak; you were simply doing your job. 

'I understand your wish to relieve the pressure on your family and your colleagues at the Embassy; I admire the fact that you think more of others than yourself. 

'You demonstrate the essence of the values of British public service. I want to stress my deep appreciation for all you have done over the last four decades.'   

Sir Kim was already expected to step down as ambassador either later this year or early next after four years in the post - viewed as a standard term in diplomatic circles. 

Speaking before Sir Kim's resignation was made public, Mr Johnson said: 'I don't think any civil servant should be thrown to the wolves nor should their advice to ministers be leaked. 

Sir Kim's decision to quit represents a major win for Donald Trump (pictured shaking hands with Sir Kim together at an event at Capitol Hill). The US President said he would no longer do business with Mr Trump

'Nor should their careers be commented on in public.' 

Mr Johnson last night said he would not be so 'presumptuous' as to talk about decisions he may make if he wins the race for Number 10 when he was asked if he would keep Sir Kim in post.

But Mr Hunt was unequivocal as he said he would not replace Sir Kim until the veteran diplomat retires. 

Mr Johnson repeatedly dodged the question over Sir Kim's future, prompting the Foreign Secretary to try to pin him down on the issue. 

Mr Hunt said: 'I will keep him until he is due to retire and I think we would like to know if you would.'

Mr Johnson replied: 'I am not going to be so presumptuous... what I will say is I, and I alone, will decide who takes important politically sensitive jobs such as the UK ambassador to the US.' 

Tom Tugendhat, the Tory chairman of the Foreign Affairs Select Committee, appeared to take a shot at Mr Johnson as he responded to the resignation by tweeting: 'Leaders stand up for their men. 

'They encourage them to try and defend them when they fail.'

Sir Alan Duncan, Mr Johnson's former deputy at the Foreign Office, said this afternoon his old boss had 'basically thrown this fantastic diplomat under the bus to serve his own personal interests'. 

Last night the Foreign Office Minister called Mr Johnson an 'utter wimp' after he failed to guarantee Sir Kim's future. 

Sir Alan tweeted: 'Am severely disappointed that Boris Johnson appears unwilling to stand up for our Ambassador to the US, our PM or our country. 

'What does that tell you? Utter wimp when the crunch comes when he should be making a stand. Err... next PM???'

The clash during an ITV Tory leadership debate came after Mr Hunt had earlier demanded Mr Trump treat the UK 'with respect' as he criticised the US President for attacking Mrs May.

Before the debate the Foreign Secretary said Mr Trump was 'disrespectful and wrong' to have called Mrs May a 'disaster' and Sir Kim a 'pompous fool' as he upped the stakes in the worsening row with the US over the leaked memos.  

The US President had described Sir Kim as a 'very stupid guy' who had been 'foisted upon the United States' and labelled Mrs May's handling of Brexit a 'mess' before then calling her 'foolish'. 

Mr Trump's comments added fuel to the fire of what was already an incredibly damaging row between the two old allies.