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 Tory Prime Minister Theresa May asked Williamson to leave the Government having "lost confidence in his ability to serve".
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.
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.
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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."
The leak from the National Security Council sparked one of the biggest mole hunts ever undertaken by Downing Street.
The inquiry followed reports that ministers had raised concerns over a plan to allow Huawei limited access to help build the UK’s new 5G network.
Williamson, aged 42, has only been Defence Secretary since late 2017, having been rewarded by May for his loyalty as chief whip.
Dubbed Private Pike because of his similarity to the Dad’s Army character, many felt he was promoted beyond his ability and was plotting to position himself as a Prime Ministerial candidate when May is forced out.
His departure will further destabilise May’s government which has lost a minster on average every six weeks since the 2017 election.
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."
Tory MP Penny Mordaunt had now been named the new Defence Secretary - the first woman to hold the post.
