Gavin Williamson has been sacked as Defence Secretary following an inquiry into the leak of information from last week's National Security Council meeting.
Downing Street said that Prime Minister Theresa May asked Mr Williamson to leave the Government having "lost confidence in his ability to serve".
Mr Williamson was immediately replaced in his role at the head of the Ministry of Defence by International Development Secretary Penny Mordaunt.
A Royal Navy reservist, she will be the UK's first female Defence Secretary and retains the job of minister for women and equalities. Rory Stewart enters the Cabinet as new International Development Secretary.
The inquiry by Cabinet Secretary Sir Mark Sedwill was launched after information from secret discussions 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 in the Telegraph as being among a small group of ministers whose warnings about Huawei's involvement were overruled by the Prime Minister.
In a statement, a Downing Street spokesman said: "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.
"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."
In a meeting with Mr Williamson on Wednesday evening, Mrs May confronted him with information which she said provided "compelling evidence" that he was responsible for the unauthorised disclosure.
And in a letter confirming his dismissal, she told him: "No other, credible version of events to explain this leak has been identified."
Mrs May said that the leak from the April 23 meeting was "an extremely serious matter and a deeply disappointing one".
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.
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.
"It has been conducted fairly, with the full co-operation of other NSC attendees," she wrote.
"They have all answered questions, engaged properly, provided as much information as possible to assist with the investigation, and encouraged their staff to do the same. 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."
Mr Williamson last week denied being responsible for the leak, issuing a statement to say: "Neither I nor any of my team have divulged information from the National Security Council."
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.
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."
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 spokesman 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."
South Staffordshire MP Mr Williamson, 42, was a surprise appointment as Defence Secretary in November 2017, after a meteoric rise which saw him enter the Cabinet without ever having served in a junior ministerial role.
He was one of Mrs May's closest allies after she made him chief whip on entering Downing Street in 2016. But during his time in the Cabinet, he showed increasing signs of independence from the PM and was widely regarded as preparing for a tilt at the top job when she stands down.
Steve Turner, assistant general secretary of Unite, said new Defence Secretary Ms Mourdant should take immediate action to make sure British warships are built in the UK.