The government's Chief Whip, Andrew Mitchell, has resigned his post after weeks of pressure over the "pleb" scandal.
Mitchell has been battered by repeated calls for him to step down after he allegedly called a police officer guarding the gates of Downing Street a pleb in a confrontation in September.
Mitchell was said to have come to the decision to resign on Thursday but wanted to sleep on it.
Mitchell denied using the derogatory terms pleb or moron but admitted in his resignation letter to Cameron that he did say: "I thought you guys were supposed to f*****g help us".
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."
Cameron accepted his resignation in a return letter which said: "I understand why you have reached the conclusion you have and why you have decided to resign from the government."
Mitchell's resignation was confirmed by 10 Downing Street in a tweet. He is expected to make a statement at the House of Commons.
Questions have been raised at the timing of his resignation as the government continues to be hit with claims that it is out of touch with ordinary people.
Mitchell's letter was announced within hours of Chancellor George Osborne being forced by a train inspector to pay £160 excess fare for travelling in a first-class compartment on a standard-class ticket.