- Video report by ITV News Political Correspondent Romilly Weeks
The head of the UK's diplomatic service has said he fears more leaks could follow that which led to the UK's ambassador to the US resigning.
Sir Simon McDonald told MPs he was "bracing" himself for further leaks, following the divulging of Sir Kim Darroch's emails which he described as "the worst breach of trust in our service in my career".
It comes as Downing Street said it was in discussions with the police as to whether any criminality has been committed when memos in which Sir Kim called the Trump administration " "inept", "incompetent" and "insecure" were leaked.
After the information was made public, US President Donald Trump tweeted that Sir Kim was "a very stupid guy", was “not liked or well thought of within the US”, and that the White House “will no longer deal with him”.
The US President also called Theresa May foolish and accused her and her team of making a "mess” of Brexit.
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- Donald Trump 'disrespectful and wrong' in attack on Theresa May and UK ambassador, says Jeremy Hunt
- Sir Kim Darroch: Who is the UK ambassador who labelled the Trump administration 'inept'?
On Wednesday, three days after the memos were leaked and as it became clear that the diplomatic spat was not going away, Sir Kim announced his resignation.
The 65-year-old had been due to step down at the end of this year but the leaked memos critical of Mr Trump have hastened his exit.
In his resignation letter on Wednesday, Sir Kim said the leaks had made it "impossible for me to carry out my role".
He praised those who he had worked with in the civil service, saying the messages of support he had received "during these difficult few days" had "brought home the depth of friendship and close ties between our two countries".
ITV News understands the comments made by Tory leadership hopeful Boris Johnson, in which he failed to give assurances that he would keep Sir Kim on if he became prime minister, added to his decision to quit.
The former foreign secretary sidestepped questions over whether he would allow Sir Kim Darroch to remain in the job until he is due to retire later this year, insisting it was “vital” the civil service is not politicised.
However his rival Mr Hunt told the ITV audience he would “certainly” keep Sir Kim in the role.
Following Sir Kim's resignation, ITV News Washington Correspondent Robert Moore said that there was anger in the UK diplomatic community in the US, in the consulates and Britain's UN mission, as well as a sense of deep disappointment.
Both towards the person who leaked the information, and also towards President Trump whose conduct they view as deeply disrespectful towards a key ally.
Sir Kim's resignation was met with deep dismay across Westminster.
During Prime Minister's Questions, Theresa May, who had been steadfast in her support of the diplomat in the wake of the leaked memos, said his resignation was a "matter of great regret".
She told the House of Commons: "This morning I have spoken to Sir Kim Darroch.
"I have told him that it is a matter of great regret that he has felt it necessary to leave his position as ambassador in Washington.
"The whole Cabinet rightly gave its full support to Sir Kim on Tuesday. Sir Kim has given a lifetime of service to the United Kingdom and we owe him an enormous debt of gratitude.
"Good government depends on public servants being able to give full and frank advice.
"I want all our public servants to have the confidence to be able to do that and I hope the House will reflect on the importance of defending our values and principles, particularly when they are under pressure."
ITV News also understands that Mrs May is seeking to fast-track the appointment of Sir Kim's successor to ensure that it is her and not the next prime minister who decides.
Labour leader Jeremy Corbyn also paid tribute in the Commons, telling MPs: "I too regret the resignation of Sir Kim Darroch.
"I think the comments made about him are beyond unfair and wrong. I think he has given honourable and good service and he should be thanked for it.
"And I think the whole House should join together in deeply regretting the feeling that he has obviously got that he must resign at this moment."
- Angus Walker explains how the hunt is on for the leak source
On Wednesday, Mr Hunt said he "profoundly regrets" the episode which led to Sir Kim's resignation. He added: "He deserves to look back upon his career as a servant of Britain with the greatest satisfaction and pride."
Mr Johnson said he too "regrets" Sir Kim's departure.
- How long will the hunt for the source of the leak take?
- Sir Kim Darroch's resignation letter in full
In a statement to Sir Simon McDonald, Permanent Under Secretary at the Foreign and Commonwealth Office, Sir Kim wrote: “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.
“I am grateful to all those in the UK and the US, who have offered their support during these difficult few days. This has brought home to me the depth of friendship and close ties between our two countries. I have been deeply touched.
“I am also grateful to all those with whom I have worked over the last four decades, particularly my team here in the US. The professionalism and integrity of the British civil service is the envy of the world. I will leave it full of confidence that its values remain in safe hands.”