Tory Owen Paterson has announced he will resign as the MP for North Shropshire.
The former Environment Secretary said he would step down after nearly 25 years in Parliament after being at the centre of a furious row over standards.
Mr Paterson was found to have misused his position as an MP to lobby for two private firms which paid him £112,000 a year.
He claimed the investigation was unfairly conducted and referenced the "torturous and inadequate" investigation in his wife's suicide last year.
The Commons Standards Committee recommended a 30-day suspension for breaching the rules - but Boris Johnson ordered Tory MPs to block it on Wednesday.
However the PM U-turned on his decision on Thursday after a furious backlash, leaving Mr Paterson facing another vote.
In a statement on Thursday, Mr Paterson said he would resign, triggering a by-election in his seat.
He said he felt his wife's suicide last year risked becoming a "political football" and his children had begged him to leave politics.
Mr Paterson said he would "remain a public servant but outside the cruel world of politics".
He said: "I have today, after consultation with my family, and with much sadness decided to resign as the MP for North Shropshire.
"The last two years have been an indescribable nightmare for my family and me.
"My integrity, which I hold very dear, has been repeatedly and publicly questioned.
"I maintain that I am totally innocent of what I have been accused of and I acted at all times in the interests of public health and safety.
"I, my family and those closest to me know the same. I am unable to clear my name under the current system.
"Far, far worse than having my honesty questioned was, of course, the suicide of my beloved and wonderful wife, Rose.
"She was everything to my children and me. We miss her everyday and the world will always be grey, sad and ultimately meaningless without her."
It comes after Downing Street confirmed that Mr Johnson had abandoned attempts to protect Mr Paterson from sanction in the wake of furious outcry.
Tory MPs backed an amendment to overhaul the disciplinary process for MPs - and block a 30-day suspension for Mr Paterson at the same time.
But the scheme fell apart when opposition parties boycotted a planned "corrupt" Tory-led committee to draw up the new system.
In farcical scenes, a new vote was due to be held next week to overturn the move.
Labour leader Keir Starmer said the Prime Minister must apologise to the nation for "this grubby attempt to cover up the misdemeanour of his friend".
“This has been an unbelievable 24 hours even by this government's chaotic standards," he said.
"Only yesterday Boris Johnson was forcing his MPs to rip up the rules on standards in public life is a truly damning indictment of this Prime Minister and the corrupt government he leads.
"Boris Johnson must now apologise to the entire country for this grubby attempt to cover up for the misdemeanour of his friend.
"This isn't the first time he's done this but it must be the last. And Boris Johnson must explain how he intends to fix the immense harm he has done to confidence in the probity of him and his MPs.”
Lord Evans, a former MI5 chief, told an Institute for Government event: "It cannot be right to propose an overhaul of the entire regulatory system in order to postpone or prevent sanctions in a very serious case of paid lobbying by an MP.
"And it cannot be right to propose that the standards system in the House of Commons should be reviewed by a select committee chaired by a member of the ruling party and with a majority of members from that same party.
"This extraordinary proposal is deeply at odds with the best traditions of British democracy."
Labour’s National Campaign Co-Ordinator Shabana Mahmood said: “I think it’s right that Owen Paterson resigned”.
But she said the scandal was more about Boris Johnson’s tactics which would “make Donald Trump blush” than Mr Paterson himself.
“Boris Johnson has shredded public confidence in standards in public life,” she said.
Boris Johnson said: “I am very sad that Parliament will lose the services of Owen Paterson who has been a friend and colleague of mine for decades.
“He has had a distinguished career, serving in two cabinet positions, and above all he has been a voice for freedom - for free markets and free trade and free societies - and he was an early and powerful champion of Brexit.
“I know that this must have been a very difficult decision but I can understand why - after the tragic circumstances in which he lost his beloved wife Rose - he has decided to put his family first.”
