Andrew Mitchell has quit as chief whip, a month after his notorious confrontation with police in Downing Street.
In a letter to the Prime Minister, Mr Mitchell said it had become clear that "I will not be able to fulfil my duties as we would both wish".
But he again denied calling the officers "plebs" after they refused to let him cycle out of the main gates.
The resignation came amid rumours that he had lost the backing of colleagues in the Whips' office - although backbenchers were broadly supportive at a meeting earlier this week.
Mr Mitchell admitted that the episode had been "upsetting and damaging" for the party, thanking David Cameron for his "loyalty". And for the first time, he publicly spelled out his version of the incident.
"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 wrote. "The offending comment and the reason for my apology to the police was my parting remark, 'I thought you guys were supposed to f***ing 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."
The timing of the departure is likely to draw allegations of cynicism, coming as Chancellor George Osborne faces widespread mockery over apparently trying to travel first class on a train with a standard ticket.
Mr Mitchell's fate is believed to have been sealed on Wednesday, when after a tumultuous PMQs deputy chief whip John Randall reportedly had to be talked out of quitting in protest at his determination to cling on.
In his letter of reply, Mr Cameron said he "understood" why he was resigning, adding: "I regret this has become necessary."