Video report by ITV News Political Correspondent Libby Wiener
Theresa May has sacked Gavin Williamson as Defence Secretary despite his "strenuous" denial that he was responsible for the unprecedented leak of information from the National Security Council (NSC).
Theresa May acted following the leak of secret discussions surrounding the Huawei 5G contract.
The NSC sees ministers meet with heads of security and intelligence agencies to hold top secret discussions.
It is thought the sacking is the first time a Cabinet minister has been sacked following a leak investigation.
The sacking was swiftly followed by calls from Labour and Liberal Democrats for a criminal investigation into whether Mr Williamson had breached the Official Secrets Act, but Downing Street made clear Mrs May now considers the matter "closed".
ITV News Political Editor Robert Peston says he has heard from Gavin Williamson who has maintained his innocence, and adds he does not understand why a police inquiry has not been launched, since leaks from the NSC are criminal offences.
So, what happened on Wednesday?
The 42-year-old learnt his fate in a half-hour face-to-face meeting in Mrs May's House of Commons office, where she told him that a leak inquiry had found "compelling evidence" of his involvement.
Mrs May summoned the Staffordshire South MP for the brutal dismissal after being briefed on the results of the inquiry shortly after Prime Minister's Questions in the Commons.
ITV News Political Editor Robert Peston tweeted that the dismissal letter sent by Mrs May to Mr Williamson was "humiliating" for him and that it "must be the end of his career in serious politics".
What has Mr Williamson said?
Mr Williamson hit back with a letter to the Prime Minister in which he cast doubt on the investigation conducted by Cabinet Secretary Sir Mark Sedwill.
"I am sorry that you feel recent leaks from the National Security Council originated in my department," Mr Williamson wrote.
"I emphatically believe that this was not the case.
"I strenuously deny that I was in any way involved in this leak and I am confident that a thorough and formal inquiry would have vindicated my position."
Mr Williamson revealed that he rejected an offer from Mrs May to resign rather than be sacked, saying that this would have sent a signal that he accepted that either he or his team was guilty.
Who will be the new Defence Secretary?
In the aftermath of the sacking, Penny Mordaunt, the International Development Secretary and Royal Navy reservist, was appointed Defence Secretary, making her the first female in British political history to take the role.
The 46-year-old said it was a "huge privilege" to work with "the best armed forces in the world".
Ms Mordaunt will retain her role as minister for women and equalities.
Prisons Minister Rory Stewart will replace Ms Mordaunt.
Speaking to ITV News after his appointment, Mr Stewart said that although he was not part of the inquiry he had "enormous confidence" the National Security adviser and the Prime Minister had followed the correct procedures in their investigation and decision to sack Mr Williamson.
What did the letter sent by Mrs May to Mr Williamson say?
In a letter to Mr Williamson, Mrs May said that the leak from the April 23 meeting was "an extremely serious matter and a deeply disappointing one".
Mrs May said it was "vital for the operation of good government and for the UK's national interest" for NSC members to be able to have "frank and detailed discussions in full confidence that the advice and analysis provided is not discussed or divulged beyond that trusted environment".
Informing Mr Williamson of his dismissal, Mrs May said she was "concerned" at the manner in which he had engaged with Sir Mark's inquiry.
Other NSC attendees had "all answered questions, engaged properly, provided as much information as possible to assist with the investigation, and encouraged their staff to do the same", she said.
But she added: "Your conduct has not been of the same standard as others'.
"In our meeting this evening, I put to you the latest information from the investigation, which provides compelling evidence suggesting your responsibility for the unauthorised disclosure.
"No other, credible version of events to explain this leak has been identified."
What caused the inquiry and sacking?
The inquiry by Cabinet Secretary Sir Mark Sedwill was launched after information from a meeting of the National Security Council about Chinese telecoms firm Huawei's involvement in the development of the UK's 5G mobile network was printed in the Daily Telegraph.
Mr Williamson was listed as being among a small group of ministers whose warnings about Huawei's involvement were overruled by the Prime Minister.
Huawei has been ostracised by a number of nations - including the US and Australia - over security doubts because of its close links to Beijing and spying concerns.
Mr Williamson last week insisted neither he nor any of his team had “divulged information from the National Security Council”.
How was the inquiry carried out?
It is understood that Sir Mark interviewed members of the NSC as well as asking them to hand over their mobile phones so they could be checked for any trace of contacts with journalists.
Are leaks from National Security Council meetings common?
While leaks from Cabinet meetings are relatively frequent, it is unprecedented for private conversations from a forum where the most senior ministers are briefed by heads of the security and intelligence agencies to reach the public.
What have opposition politicians said?
Labour deputy leader Tom Watson said: "If he has leaked from the National Security Council, Gavin Williamson should be prosecuted under the Official Secrets Act. And he should forgo his ministerial severance pay."
Labour's shadow defence secretary Nia Griffiths added: “The Tories are in chaos and incapable of sorting out their own crisis.
"Conservative infighting has undermined the basic functioning of government, and has now potentially put security at risk.
"The police must urgently investigate.”
And Liberal Democrat leader Sir Vince Cable said: "This story cannot begin and end with dismissal from office.
"What is at stake is the capacity of our security services to give advice at the highest level.
"This must now be referred to the Metropolitan Police for a thorough criminal investigation into breaches of the Official Secrets Act."
Change UK's defence spokesperson Mike Gapes said: "This is an extraordinary development following an unprecedented leak from the National Security Council and I would now expect a criminal investigation to follow.
"This is yet another sign of a dysfunctional chaotic Conservative government.
"Our politics is broken, it's time for a change."
How has Downing Street summed up the day's events?
"The Prime Minister has this evening asked Gavin Williamson to leave the Government, having lost confidence in his ability to serve in the role of Defence Secretary and as a member of her Cabinet," a Downing Street spokesperson said.
"The Prime Minister's decision has been informed by his conduct surrounding an investigation into the circumstances of the unauthorised disclosure of information from a meeting of the National Security Council.
"The Prime Minister thanks all members of the National Security Council for their full cooperation and candour during the investigation and considers the matter closed."