British Cabinet Office minister Gavin Williamson has resigned, saying a series of allegations against him were "becoming a distraction for the good work this government is doing".
Mr Williamson was facing a series of inquiries into his conduct as fresh allegations emerged about his "unethical and immoral" behaviour.
Downing Street was carrying out a fact-finding exercise following claims that he told a senior civil servant to "slit your throat".
The Conservative party and parliament's bullying watchdog were also looking into allegations made by former chief whip Wendy Morton.
Pressure continued to mount on Mr Williamson throughout the day as his former deputy Anne Milton alleged he used intimidation and threatening tactics while he was chief whip.
The senior civil servant who claims to have been bullied by Mr Williamson when he was defence secretary has reportedly submitted a formal complaint to Parliament's Independent Complaints and Grievance Scheme (ICGS).
British Prime Minister Rishi Sunak's decision to give a Cabinet-level role to the twice-sacked minister has led to questions about his judgment from critics.
Earlier, Downing Street said that Mr Sunak still maintained confidence in his ally, but indicated the prime minister would act if necessary.
In his resignation letter, Mr Williamson told Mr Sunak: "As you know, there is an ongoing complaints process concerning text messages I sent to a colleague.
"I am complying with this process and I 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 under way and clear my name of any wrongdoing."
— Gavin Williamson (@GavinWilliamson) November 8, 2022
Labour deputy leader Angela Rayner 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 judgment 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."
Expletive-laden texts
In the latest disclosures, Ms Milton accused Mr Williamson of seeking to use an MP's financial difficulties as leverage against them and sending an expletive-laden email about a female civil servant.
Ms Milton, who was deputy chief whip from May 2015 to June 2017, described his behaviour as "unethical and immoral", claiming: "I think he feels that he's Francis Urquhart from House Of Cards."
Ms Milton, who was stripped of the Tory whip during the Brexit rows in 2019 and subsequently lost her seat, told Channel 4 News: "I got the impression that he loved salacious gossip, and would use it as leverage against MPs if the need arose."
She told the broadcaster that Mr Williamson had a rant about civil servants in 2016 in response to a female official asking why a minister had to change travel plans for a vote.
"Always tell them to f*** off and if they have the b******* to come and see me," he said in an email, according to Ms Milton.
"F****** jumped up civil servants."
She said that when the whips' office gave some financial assistance to an MP, Mr Williamson told her that when she handed over the cheque she should make sure "he knows I now own him".
She questioned Mr Sunak's decision to give Mr Williamson a ministerial job, saying: "I think (at) best it was probably a bit naive. I don't know that there are many people that would hang out the bunting to see Gavin Williamson back in government."
The claims by Ms Milton follow allegations that Mr Williamson clashed with a civil servant while he was defence secretary.
Mr Williamson denied the official's allegation of a campaign of bullying, but did not deny making the remarks, including telling the civil servant to "jump out of the window".
The Guardian, which first reported the allegations, said a complaint had now gone to the ICGS.
In a statement, the complainant said they had an "incredibly difficult period" working for Mr Williamson at the MoD and that the alleged bullying had had "an extreme impact" on their mental health.
Pressure on Sunak over appointment
Downing Street was understood to be undertaking a fact-finding exercise and considering how to respond to the allegations.
The prime minister's official spokesman earlier said: "Those are serious allegations that have come in."
In a statement following the Guardian report, Mr Williamson said: "I strongly reject this allegation and have enjoyed good working relationships with the many brilliant officials I have worked with across government."
Meanwhile, the PA news agency understands that former chief whip Ms Morton has referred her complaint to parliament's bullying watchdog, while an internal Tory party inquiry is ongoing.
Mr Williamson's newly revealed ministerial responsibilities included the Geospatial Commission, the Government Communications Service and the "Great Campaign" to promote the UK.
The prime minister had been under sustained pressure over his decision to bring his ally back into government despite knowing about the complaint made by Ms Morton.
The Times, which first disclosed that she has referred her complaint to Parliament's Independent Complaints and Grievance Scheme, said she made the move over fears of a "whitewash" by the Conservative Party.
In a series of texts peppered with swear words, Mr Williamson accused Ms Morton of seeking to "punish" MPs out of favour with then-premier Liz Truss by excluding them from the Queen's funeral, warning: "There is a price for everything."
The Liberal Democrats demanded an independent inquiry into his conduct so the Tories do not "mark their own homework".
Mr Williamson, who was knighted after being nominated for the honour by Boris Johnson earlier this year, is a divisive figure at Westminster, where he is viewed with suspicion by many Tory MPs because of his reputation as an inveterate plotter.
He was sacked first by former PM Theresa May as defence secretary in 2019 for leaking details of a National Security Council meeting, and then by Mr Johnson as education secretary over the Covid-19 A-levels debacle.
However, he was regarded as a key figure in Mr Sunak's campaign over the summer to become party leader.
