Sir Gavin Williamson has resigned from government over accusations of bullying, saying he will "clear my name of any wrongdoing".
His departure came shortly after an ex-civil servant, who claimed Sir Gavin told them to "slit your throat", made a formal complaint against the Cabinet Office minister.
In his resignation letter shared on Twitter, Sir Gavin said he "refutes the characterisation of these claims", but that the allegations were "becoming a distraction for the good work this government is doing for the British people".
As a result, he had "decided to step back from government" and would "fully comply with the complaints process".
He later returned to Twitter to "dispel any speculation" by saying he would not be taking any severance pay.
It is the third time Sir Gavin has resigned from a cabinet post.
In his response, Rishi Sunak, the prime minister, said he accepted the resignation "with great sadness", but added: "I support your decision to step back and understand why you have taken it."
Labour's deputy leader Angela Rayner said the events had shown "yet another Tory government has descended into chaos".
Allegations emerged over the weekend that Sir Gavin had sent abusive texts to Wendy Morton, the former chief whip, complaining he had been excluded from the Queen's funeral.
Ms Morton made a complaint to the Conservative Party about the messages, but earlier today confirmed she was reporting him to the Independent Complaints and Grievance Scheme (ICGS), parliament's independent watchdog.
'Slit your throat'
A further allegation surfaced on Monday when a former Ministry of Defence official claimed Sir Gavin had told them to "slit your throat".
On another occasion, the ex-civil servant said, he had told them to "jump out of the window" in what they described as a sustained campaign of bullying.
Less than two hours before Sir Gavin quit, that former official confirmed they would also go to the ICGS as the incidents had had an "extreme impact" on their mental health.
Writing to Mr Sunak, who appointed him to his cabinet two weeks ago and had stood by his minister throughout the week, Sir Gavin said: "As you know, there is an ongoing complaints process concerning text messages I sent to a colleague [Ms Morton]. I am complying with this process and have apologised to the recipient for those messages.
"Since then there have been other allegations made about my past conduct. I refute the characterisation of these claims but I recognise these are becoming a distraction for the good work this government is doing for the British people.
"I have therefore decided to step back from government so that I can comply fully with the complaints process that is underway and clear my name of any wrongdoing."
'Resigning with real sadness'
Sir Gavin added that it was "with real sadness" that he offered his resignation, but that he would offer Mr Sunak his "full and total support from the backbenches", and was "proud" to have worked for his government.
Writing in response, Mr Sunak said Sir Gavin's "commitment to successive Conservative governments and the party over the years has been unwavering".
After saying he supported the decision, the PM added: "I would like to thank you for your personal support and loyalty. I know you will continue to represent your constituents with diligence and care."
'Damning reflection of weak PM'
Labour's Ms Rayner claimed Mr Sunak had appointed Sir Gavin "with full knowledge of serious allegations about his conduct and repeatedly expressed confidence in him", adding the events of the past week had been "a damning reflection of a weak prime minister".
She said: "This is yet another example of Rishi Sunak's poor judgement and weak leadership. It is clear that he is trapped by the grubby backroom deals he made to dodge a vote, and is incapable of putting country before party."
Sir Gavin served as Theresa May's chief whip from 2016 and then as defence secretary from November 2017 until May 2019, but was fired over a row around a leak from government.
He was hired back by Boris Johnson as education secretary two months later, but was again ousted in September 2021 after much criticism of his handling of the brief during the pandemic.
Daisy Cooper, the Liberal Democrat deputy leader, said: "This should be the third and final time Gavin Williamson is forced out of the Cabinet.
"Rishi Sunak has serious questions to answer about why he appointed Gavin Williamson, then stood by him instead of sacking him. His promise to lead a government of integrity has now been left in tatters."