Labour leader Ed Miliband has branded the Prime Minister "weak" after Andrew Mitchell's resignation as the Government's Chief Whip.
Mr Mitchell, who has been replaced by veteran MP Sir George Young, stepped down the wake of claims he called police officers "plebs" in Downing Street.
Labour Ed Miliband used the development to lash out at David Cameron, saying: "Andrew Mitchell didn't go because of what he did, he didn't go because of David Cameron's shame at what he did - he went because David Cameron was too weak to hang on to him.
"The problem is it's part of a pattern with this Government - a pattern of one rule for those at the top and another rule for everybody else."
Writing to the Prime Minister, Mr Mitchell said: "It is with enormous regret - not least because of the tremendous support and loyalty you have shown me during recent weeks - that I am writing to resign as your Chief Whip.
"Over the last two days it has become clear to me that whatever the rights and wrongs of the matter I will not be able to fulfil my duties as we would wish. Nor is it fair to continue to put my family and colleagues through this upsetting and damaging publicity.
"I have made clear to you - and I give you my categorical assurance again - that I did not, never have and never would call a police officer a "pleb" or a "moron" or used any of the other pejorative descriptions attributed to me."
He went on: "The offending comment and the reason for my apology to the police was my parting remark 'I thought you guys were supposed to f****** help us'.
"It was obviously wrong of me to use such bad language and I am very sorry about it and grateful to the police officer for accepting my apology."
In a letter to Mr Mitchell, Prime Minister David Cameron said: "I understand why you have reached the conclusion you have, and why you have decided to resign from the Government.
"I regret that this has become necessary, and I am very grateful for all you have done, both in Government and in Opposition - as well as for the kind words in your letter."
Mr Cameron added: "As you have acknowledged, the incident in Downing Street was not acceptable and you were right to apologise for it."
Paying tribute to his work, Conservative MP Zac Goldsmith tweeted: "Andrew Mitchell should be remembered as the best Development Secretary this country has had."
Michael Fabricant MP said online: "I spoke to Andrew yesterday about this and I was frank. This has been playing out like a long Greek tragedy and I hope that now he will take a break and rebuild his strength. He is a very able individual and I am sure he will return to public life in due course."
Labour's shadow cabinet office minister Michael Dugher MP said: "After weeks in complete denial, Andrew Mitchell has finally bowed to public pressure.
"What people will want to know is why, when the entire country could see that what Andrew Mitchell did was wrong, the Prime Minister totally failed to act."
Sky's Political Editor Adam Boulton said it was clear there was "very little support" for Mr Mitchell from fellow Tories.
"Even those who sympathised with him for losing his temper felt that he was damaging the Government," he said.
"There has been a lot of unrest from the present whips office about how they're going to maintain discipline with a Chief Whip who has damaged his own authority and reputation in this way."
The Chairman of the Police Federation of England and Wales, Paul McKeever, said: "It is not good to see anyone fall from public office but the decision by the Prime Minister to accept Andrew Mitchell's resignation seemed almost inevitable.
"Andrew Mitchell has apologised to our Metropolitan Police colleague and our colleague has accepted the apology. We hope this matter is now closed."