Alun Cairns has resigned as Secretary of State for Wales after being accused of "brazenly lying" about his knowledge of an allegation that a Tory candidate had sabotaged a rape trial.
Mr Cairns is accused of knowing former aide Ross England made claims about a rape victim's sexual history during a trial in April 2018, which led to the case collapsing.
The Welsh Secretary has denied knowing about the collapsed case, although BBC Wales were leaked email which appeared to show Mr Cairns was made aware of the allegations.
Mr England is standing to be a Conservative candidate for the Welsh Assembly elections in 2021 but has faced calls to step aside.
In a letter to the prime minister, Mr Cairns wrote: "You will be aware of allegations relating to the actions of a party employee and candidate for the Welsh Assembly elections in the Vale of Glamorgan.
"This is a very sensitive matter, and in light of continued speculation, I write to tender my resignation as Secretary of State for Wales.
"I will cooperate in full with the investigation under the Ministerial Code which will now take place and I am confident I will be cleared of any breach or wrong doing."
In a letter replying to Mr Cairns, Boris Johnson thanked Mr Cairns for his resignation and his efforts in government.
Mr Johnson wrote: "In particular, I would like to put on record my gratitude for all the support you have given to this Government in ensuring we honour the commitment to the people that we leave the European Union.
"Given your long service as Secretary of State, you can be proud of your record of delivery for the people of Wales, in particular in ensuring the abolition of tolls on the Severn bridges.
"This follows an unstinting record of service to the Party in Wales with over a decade as Assembly Member for South Wales West where you were a vocal critic of the Welsh Labour government."
Despite resigning from his role in the government, Mr Cairns is expected to remain a parliamentary candidate for the Vale of Glamorgan.
Mr Cairn's resignation from the Cabinet comes after the Conservative Party has been forced onto the back-foot at the start of the election period.
Welsh Labour and Plaid Cymru called on Mr Cairns to step aside as a parliamentary candidate for the Vale of Glamorgan in the December 12 general election.
Christina Rees, shadow secretary of state for Wales said: "Alun Cairns stepping down as secretary of state is far from the end of the matter, and is a shoddy halfway house that will fool nobody.
"He has still not explained his behaviour and still not addressed the grave issues raised by the leaked emails yesterday.
"Worse still, neither he nor any senior Welsh Tory have apologised to one person who most deserves it - the victim herself.
"He should do the right thing - apologise, and step down as a candidate."
Plaid Cymru MP Liz Saville Roberts said: "I hope that Alun Cairns' resignation as secretary of state proves to be some solace to the woman at the centre of this case, who still has received no apology from the Conservative Party.
"Mr Cairns' conduct proved unquestionably that he is not fit to hold ministerial office.
"You simply cannot be complicit in the attempted cover up of sabotaging a rape trial and hope to get away with it.
"Such conduct is neither befitting of a government minister, nor of a member of parliament.
"Mr Cairns should do the honourable thing and withdraw from the election - and if he doesn't, the Conservative Party should insist he withdraws."
Commenting on Mr Cairns, Ms Swinson added: "And I think there are real questions to be asked about why it has taken him so long to come to this conclusion."