Sir Gavin Williamson has resigned after a series of bullying accusations were made against him, including allegedly telling an aide to "slit your throat".
Prime Minister Rishi Sunak appointed Sir Gavin to his Cabinet as Minister of State without Portfolio after arriving in No10 last month.
But reports about his alleged conduct have led to the former Defence Secretary stepping down tonight.
It is the first major blow to the new PM's premiership.
In recent days Sir Gavin faced claims of boasting he "owned" a Tory MP facing financial difficulty, allegedly telling a civil servant to "slit your throat”, and sent expletive-ridden messages to the party's ex-Chief Whip.
Senior Tories called on Mr Sunak to oust him from Cabinet to end the “advent calendar” of daily sleaze allegations engulfing No10.
In a letter posted on Twitter, Sir Gavin said while he refuted the "characterisation" of claims against him, he recognised "these are becoming a distraction for the good work this government is doing".
The MP for South Staffordshire added he would comply with the complaints process and "clear my name of any wrongdoing".
The resignation will raise questions about Mr Sunak's judgement after he brought Sir Gavin back into Government - despite the Tory MP being sacked by both Theresa May and Boris Johnson in recent years.
In quitting, shamed Sir Gavin, meanwhile, could have trousered an £8k golden goodbye - more than six times what he earned on the job.
But he tweeted tonight to say he will turn it down.
It emerged on Tuesday one ex-Ministry of Defence official, who alleged Sir Gavin had told them "jump out of the window" while Defence Secretary, reported the senior Tory to the parliamentary authorities for investigation.
In a statement, the former civil servant said:"This was an incredibly difficult period for me. Gavin Williamson's words and the actions had an extreme impact on my mental health.
"Government ministers have a code of conduct that they are required to follow. Any form of abuse is unacceptable".
Over the weekend, leaked messages from Sir Gavin to Wendy Morton, the Tories' Chief Whip under Liz Truss, showed him accusing her of using the Queen's death to "punish" senior MPs who were out of favour with the leadership.
A leaked exchange of expletive-ridden messages concluded with him saying: "Well let's see how many more times you f*** us all over. There is a price for everything."
Former minister Anne Milton accused Sir Gavin of using MPs’ mental and physical health problems as “leverage” as he attempted to control them.
“I do remember him asking me to give the MP in question the cheque,” she told Channel 4 News.
“He waved it under my nose and said, 'Make sure when you give him this cheque, he knows I now own him’.”
Addressed to the PM, Sir Gavin's letter said: "As you know, there is an ongoing complaints process concerning text messages I sent to a colleague. 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.
"It is with real sadness that I tender my resignation, but I want to take this opportunity to offer my full and total support from the back benches. I am incredibly proud to have worked with you in government over the last few years and during the campaign."
Angela Rayner, Deputy Leader of the Labour Party, said: “This is a damning reflection of a weak Prime Minister.
"Rishi Sunak appointed Gavin Williamson with full knowledge of serious allegations about his conduct and repeatedly expressed confidence in him.
“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.
“As families struggle during a cost of living crisis made in Downing Street, yet another Tory government has descended into chaos.”
Also responding to the news, Liberal Democrat Deputy Leader Daisy Cooper MP 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 Willlamson, then stood by him instead of sacking him.
"His promise to lead a government of integrity has now been left in tatters.
"We now need to know what Rishi Sunak knew about these shocking allegations and when, through a full independent inquiry.
"People deserve so much better than this endless Conservative chaos."
Disgraced Sir Gavin is entitled to £7,920 in severance pay, our research suggests, despite only lasting two weeks in Cabinet.
All ministers who leave the government can claim a quarter of their annual ministerial salary no matter what the reason - even if they resign over bullying claims.
The only exception is they must not take another ministerial job within three weeks of leaving, like Suella Braverman.
In his two weeks as Cabinet Office minister he earned £1,215 - but his job description was only confirmed hours before he quit.
Sir Gavin was a Minister of State who “attended Cabinet”.
Past precedent for this role suggests it carries a salary of £31,680. Ministers of State who are “members of Cabinet” get a higher salary of £67,505.
However, he later confirmed he will turn down his severance pay.
He tweeted: “To dispel any speculation, I want to make it clear that I will not be taking any severance. This is taxpayers’ money and it should go instead toward the Government‘s priorities like reducing the NHS’s waiting lists.”
In response to Sir Gavin's resignation, Mr Sunak's official statement from No10 said: "Thank you for your resignation letter.
"It is with great sadness that I accept your resignation.
"I know your commitment to successive Conservative governments and the party over the years has been unwavering.
"I support your decision to step back and understand why you have taken it.
"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.
"I also want to express my gratitude for the work you have done for this Government."
