The ex-Work and Pensions Secretary called a press conference to defend his conduct after weeks of controversy over his links with technology firm DNA Bioscience and other firms.
"I'm guilty of a mistake and I'm paying the price of it," he told reporters, after accepting he should have consulted an advisory committee before taking up new jobs after leaving office for the first time in December.
His resignation was announced at 11am today and Prime Minister Tony Blair told the Commons an hour later that his close ally "goes, in my view, with no stain of impropriety against him whatsoever".
Downing Street insisted the minister resigned and was not sacked.
Mr Blunkett said: "What I am clear about is that I have made a mistake."
He said that correspondence he had in March with the chairman of the Advisory Committee on Business Appointments, Lord Mayhew, had led him to the erroneous belief that it was a voluntary matter whether he sought the committee's advice or not.
"Was I at fault not writing to the committee?" he asked.
"Yes, I was. It was the same fault on three occasions arising from the same misunderstanding by me.
"I have to take the consequences of that, which is why I am standing down today."
But he added: "Did I do anything wrong in buying the holding in DNA Bioscience? No I didn't.
"Did I declare that holding? Yes I did."
Mr Blunkett said he called Downing Street at 8am and asked to see the premier "and Tony asked me to stay" in the Government.
But on leaving No 10 to attend a select committee hearing at the Commons, said Mr Blunkett, he realised his position was untenable.
The Sheffield Brightside MP told reporters: "I step down today precisely to protect the Government from diversion from the policies we are carrying out.
"I don't think you need to be a genius to realise that the events not just of the last few days but of the last few weeks have been a diversion."
He said he defended a free press but dismissed some claims about him as "lies" and urged the media to leave his family and friends alone.
Earlier, Mr Blair told MPs: "I would like to say that whatever mistakes my Right Honourable Friend has made, I have always believed and believe now he is a decent and honourable man."
He defended his decision not to sack Mr Blunkett over his failure to consult an independent committee about three jobs he took up after leaving Government last December.
Mr Blair said he had investigated the matter and "I could discover no impropriety".
He said there had been "a frenzy" surrounding Mr Blunkett which made it impossible for him to continue.
But events this morning bordered on farce as the committee of MPs due to quiz Mr Blunkett on his department's work first had to postpone and then abandon their session - having been told by their chairman that Mr Blunkett had quit, nearly an hour before official confirmation came.
Speculation at Westminster was that Mr Blunkett had been fired but allowed to dictate the terms of his own departure.
No 10 firmly denied that interpretation, insisting repeatedly that it was Mr Blunkett alone who made the decision to go.
And Mr Blunkett denied at his press conference that he had been ordered to quit, insisting it was his decision.
Mr Blunkett told the press conference that having investments and holding shares"is not a crime". But he said declaring them was " imperative".
He insisted he had declared his holding and had also ensured there was a firewall between him and any decisions to be taken over the Child Support Agency in relation to DNA testing.
"I am guilty of a mistake and I am paying the price for it," he said.
"I make no bones about saying that is my fault and I stand by it."
Mr Blunkett said he was proud of his achievements in government.
He added: "I am deeply sorry for the embarrassment that I have caused to the Prime Minister... It is the Prime Minister that some people want to target."I wish to support him."
Mr Blunkett went on: "I hope in future, not of course to return to government, but to play a part in public life, in using the experience I have gained over 40 years as an elected representative to be able to improve and continue improving the life chances and the well-being of the people I serve."
Mr Blunkett insisted nothing new had emerged and "nothing has changed from the weekend".
He said his only mistake was in not writing to the chairman of the committee. "I clearly had breached the ministerial code".
"When I say I have done nothing wrong, what I mean is I have actually not in office taken any steps, been involved in any discussions, had any talks, or misused my position in any way," he said.
"Everything that has been revealed has shown, with this one exception, that I have not committed any offence that will reduce Tony Blair's confidence in me as a minister."
Mr Blunkett said that it was while he was returning from Downing Street toPortcullis House to give evidence to the committee that he had finally decidedthat he could not continue in office.
"When you have been in politics as long as I have, you can smell and feel when it is time to step away and I felt that between Downing Street and Portcullis House," he said.
He said that media "revelations" about his private life emanating from publicist Max Clifford were "lies" which he would deal with at a future date.
Asked whether he had ever suspected that he was being bugged, he replied: "On my walks in Derbyshire I have even suspected at times that the birds in the trees were wired up."
Mr Blunkett was asked if the publicity he had received over the past few weeks had led him to review his diaries in order to ensure he had a precise recollection of his activities and had declared everything that he should have declared.
He said: "Yes. The scrutiny of my life led me to review everything that I had done and that included the Register of Members' Interests.
"It had been the practice that people register within a month. Some of those things I hadn't registered within a month because it fell within the prorogued period .125during the general election.375.
"I ought to make it absolutely clear that twice I tried to register within the prorogued period of Parliament, and I was told that I couldn't register because I was neither an MP nor a minister.
"I have a record of that, just so that there's no continuing doubt."
Mr Blunkett called on the media and public not to judge politicians over events in their private and romantic life.
"What I had was precious to me and remains so, and I have no regrets about what happened years ago," he said, referring to his former relationship with Kimberly Quinn.
"I regret not writing to the Advisory Committee, but unless we understand that the private and the public are separate, we will destroy our democracy in Britain."
Asked if he had any more interests to disclose which are not yet in the publicdomain, Mr Blunkett said: "No, not at all.
"I'm not entirely sure what is in the public domain. There's certainly quite a lot of the things I've registered which are not in the public domain, despite the scrutiny.
"Everything I did while I was out of office is registered and will appear in the printed register."
Mr Blunkett indicated that he had not yet sold the £15,000-worth of shares in DNA Bioscience which are held in a trust for his sons and which he said on Monday that he was planning to sell.
He said he would talk with his sons "in a week or two's time" about what to do with the shares, which City insiders believe may increase in value to as much as £300,000 when the company floats on the stock market.
He said he had not yet decided on his future plans, but added: "I will reflect on what I can do in terms of my future and the contribution I can make publicly and privately."
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