After a month of intense political pressure Andrew Mitchell finally gave up the fight to keep his job last night after admitting he could “no longer fulfil his duties” in the wake of his abuse of a Downing Street policeman.

After a month of intense political pressure Andrew Mitchell finally gave up the fight to keep his job last night after admitting he could "no longer fulfil his duties" in the wake of his abuse of a Downing Street policeman.

In a letter announcing his decision to resign, the former Chief Whip again denied he had called the officer a "pleb" but admitted he had been unprofessional in his conduct. He also admitted for the first time that he had sworn at the officer concerned. "Whatever the rights and wrongs of the matter I will not be able to fulfil my duties as we would both wish," he said. "Nor is it fair to continue to put my family and colleagues through this upsetting and damaging publicity."

He continued: "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. The offending comment and the reason for my apology 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."

David Cameron has now been forced to replace Mr Mitchell only a month after promoting him from his previous job as International Development Secretary. Last night it was announced that his position would be taken by Sir George Young, 71, who was dropped as Leader of the Commons in the reshuffle.

Mr Mitchell is expected to make a resignation statement to the House of Commons next week.

He made the decision to go on Thursday night and contacted Downing Street yesterday asking for a private meeting with Mr Cameron.

It was arranged that he would drive down to the Prime Minister's country residence at Chequers, where he told him of his decision to step down.

Mr Mitchell is understood to have concluded his position was untenable after a meeting of Tory backbenchers on Wednesday night.

Four MPs demanded he step down, while others expressed concern at the damage the affair was doing to the image of the party following a concerted campaign by the Police Federation.

The Deputy Chief Whip, John Randall, had also reportedly been talked out of quitting in protest at his boss's determination to stay in his job.

Sources close to Mr Mitchell said he had taken "extensive soundings" from backbench MPs and felt in particular that he had lost the confidence of the 2010 intake of Conservatives – who make up half the Parliamentary party.

The former Tory whip Michael Fabricant said he had urged Mr Mitchell to step down. "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," he said.

In his letter accepting Mr Mitchell's resignation, Mr Cameron held open the possibility that he might be able to return. "I regret that this has become necessary," he said.

Paul McKeever, chairman of the Police Federation of England and Wales, said: "Andrew Mitchell has apologised to our Metropolitan Police colleague and our colleague has accepted the apology. We hope this matter is now closed."