Maria Miller has said she takes "full responsibility" for her decision to stand down as culture secretary, saying that she did not want to become a "distraction" from the achievements of the government.
She said she had hoped to carry on in office after she was cleared of the central expenses allegation against her by the Commons Standards Committee but it had become clear it was impossible to do so.
"This has been a really difficult 16 months. Because I was cleared of the central allegation made about me by a Labour Member of Parliament, I hoped that I could stay. But it has become clear to me that it has become an enormous distraction," she said. "It is not right that I am distracting from the incredible achievements of this government."
Asked if she had been pressurised into quitting, a clearly emotional Miller said: "I take full responsibility for my decision to resign. I think it is the right thing to do to remove what has become really an unhelpful and very difficult distraction for colleagues."
Asked also whether she believed she had been the victim of a media witch hunt because of her role in implementing the recommendations of the Leveson Inquiry on press standards, she said: "I take full responsibility for the situation. I fully accept the findings of the parliamentary standards report. This is about that."
In a letter to David Cameron, Miller said the controversy over her expenses "has become a distraction from the vital work this government is doing".
The prime minister is said to have been "sorry" to receive her resignation. In response to the former cabinet member, Cameron wrote that Miller could be "extremely proud" of her work in government.
The reluctant-sounding PM added: "I hope that you will be able to return to serving the government on the frontbench in due course, and am only sad that you are leaving the government in these circumstances."
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Her resignation follows days of mounting public and political pressure on the MP to quit and signs she was losing support among Tory colleagues despite Cameron's backing.
Labour MP John Mann, who made the allegations against Miller which triggered the standards investigation, said: "About time too."
He told BBC Radio 4's Today: "Maria Miller should have resigned immediately and when she didn't resign, Mr Cameron should have shown a bit of leadership and sacked her."
He said the case had shown the present system - which saw a cross-party committee of MPs dramatically reduce the sum an independent watchdog said Mrs Miller should repay - was "dead and buried".
"I would like to see David Cameron announce today at Prime Minister's Questions that that system is going to go immediately and there will be no more self-regulation of MPs by MPs."
Miller had previously admitted she "let people down" as she struggled to cling on to her Cabinet job amid a Tory backlash in response to the way she approached parliamentary standards commissioner Kathryn Hudson's probe into her accommodation expenses.."
While Conservatives broke ranks to express concern about Cameron's determination to cling on to his minister, allies launched a furious counter-attack accusing the press of a "witch-hunt" because of her role in shaping a new regulatory system.