David Blunkett has said he is "deeply sorry" for the embarrassment he has caused Tony Blair, after he resigned as work and pensions secretary.
He said he was guilty of making a mistake on three occasions and was now "paying the price for it".
Tony Blair described Mr Blunkett as a "decent and honourable man".
Mr Blunkett quit after breaking the ministerial code of conduct over paid work he took while out of the Cabinet. John Hutton is to replace him.
Michael Howard said the events showed a "haemorrhage" of Mr Blair's authority.
David Blunkett chose to resign because he had reached the conclusion that the position was untenable and that he would be unable to continue in his position
Prime Minister's official spokesman
Mr Blunkett was previously forced to step down as home secretary in December 2004.
At the centre of the recent controversy was Mr Blunkett's two-week directorship of DNA Bioscience before May's election, while he was out of the Cabinet.
Mr Blunkett broke ministerial rules by taking that job without consulting an independent committee which advises former ministers on whether they should take up jobs.
At a press conference in central London, Mr Blunkett told reporters he had been considering quitting since last week, but made his mind up to go on Wednesday "to protect the government".
'My fault'
He had been due to appear before the Commons work and pensions committee on Wednesday morning.
But instead he went to Downing Street to tell Mr Blair of his decision to resign.
"What I am clear about is that I have made a mistake," he said.
He should have consulted an advisory committee before taking up new jobs after leaving office for the first ...