Damian Green, one of Theresa May's closest allies, has been sacked from the cabinet after an inquiry found he had breached the ministerial code.
He was "asked to quit" after he was found to have made "inaccurate and misleading" statements about what he knew about claims pornography was found on a computer in his office in 2008.
In his resignation letter, Mr Green apologised for his actions.
BBC's Laura Kuennsberg said the PM "had little choice but to ask him to go".
In her written response, Mrs May expressed "deep regret" at his departure.
Mr Green, who as first secretary of state was the PM's deputy, had been under investigation regarding allegations of inappropriate conduct.
He denied suggestions that he made unwanted advances to a female journalist, Kate Maltby, in 2015 and viewed pornography on a computer in his Commons office in 2008.
An official report by the Cabinet Office found that statements he had made about being unaware pornographic material had been found on his computer were "inaccurate and misleading" and as such fell short of the ministerial code.
The report also found that although there were "competing and contradictory accounts of what were private meetings" between himself and Ms Maltby, the investigation found her account "to be plausible".
Image copyrightELIZABETH HANDY
Her parents, Colin and Victoria Maltby, said in a statement they were not surprised to find that the inquiry found Mr Green to have been "untruthful as a minister, nor to that they found our daughter to be a plausible witness".
They praised their 31-year-old daughter for her courage in speaking out about the "abuse of authority".
Ms Maltby is not commenting on Mr Green's resignation until she receives more details from the Cabinet Office.
In his letter, Mr Green said he accepted statements he made about what he knew about the pornography investigation could have been "clearer", conceding his lawyers had been informed about the original discovery in 2008 and the police had raised the matter within him in a phone call in 2013.
"I apologise that my statements were misleading on this point," he said.
May a lonelier figure tonight
Image copyrightPADamian Green has never been a politician with a huge public persona, or even a hugely well-known character.
But he was an extremely important ally of Theresa May. Not just a political friend but a genuine one, close to her for decades.
The government, so the joke in Westminster goes, has become "weak and stable", with number 10 taking back some control of the agenda in recent weeks.
So it is not likely that Mr Green's exit will suddenly unleash another bout of turmoil.
But the prime minister clearly took this decision very seriously.
She is a politician who guards her views, her own persona very closely. To lose one of the few who understood her, who she trusts, leaves her a lonelier figure tonight.
Mr Green's political future has been in question since journalist and Conservative activist Ms Maltby suggested he had behaved inappropriately towards her in an article last month for the Times.
She claimed the minister "fleetingly" touched her knee in a pub in 2015 and in 2016 sent her a "suggestive" text message which left her feeling "awkward, embarrassed and professionally compromised".
Mr Green, who is an acquaintance of the journalist's parents, said the claims were "hurtful" and "completely false".
But they were referred to the Cabinet Office for investigation by a top civil servant - who is examining other claims that emerged during a swirl of allegations about harassment and other misconduct at Westminster.
The inquiry was subsequently expanded to consider claims that legal pornography was found on a computer removed from Mr Green's office in the House of Commons in 2008.
The computer was one of a number of possessions seized by the police during a controversial inquiry into the leaking of official documents by a civil servant to Mr Green, at the time a shadow Home Office minister under David Cameron.
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