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 over what he knew about claims pornography was found on his office computer in 2008.
He also apologised for making writer Kate Maltby feel uncomfortable in 2015.
Laura Kuenssberg said the PM “had little choice but to ask him to go”.
The BBC’s political editor said the departure of a close friend left Mrs May a “lonelier figure”.
Mr Green, 61, who as first secretary of state was effectively the PM’s deputy, is the third cabinet minister to resign in the space of two months – Sir Michael Fallon and Priti Patel both quit in November.
In her written response, Mrs May expressed “deep regret” at Mr Green’s departure but said his actions “fell short” of the conduct expected of a cabinet minister.
Like Mrs May, Mr Green campaigned for Remain in last year’s EU referendum and had been a leading voice in Cabinet for a “softer” Brexit.
He had been under investigation regarding allegations of inappropriate behaviour towards journalist and Tory activist Ms Maltby. He denied suggestions that he made unwanted advances towards her in 2015.
He also denied that he had either downloaded or viewed pornography on a computer removed from his Commons office in 2008 and said police had “never suggested to me that improper material was found”.
In his resignation letter, Mr Green said statements he made about what he knew about the pornography could have been “clearer”, conceding that his lawyers had been informed by Met Police lawyers about their initial discovery in 2008 and the police had also raised the matter with him in a phone call in 2013.
“I apologise that my statements were misleading on this point,” he said.
Health Secretary Jeremy Hunt told BBC Radio 4’s Today programme Mr Green had “lied” about “a particular incident” and that was why he had to go but it was a “sad moment”.
Asked if his departure left Theresa May more isolated, he said “leadership is lonely” but she had shown “extraordinary resilience in very challenging circumstances” and was someone “who is capable of taking very difficult decisions”.
An official report by the Cabinet Office found that public statements he made relating to what he knew about the claims were “inaccurate and misleading” and constituted a breach 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”.
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 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.
Analysis by BBC political editor Laura Kuenssberg
Damian 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.
