Andrew Mitchell resignation: A setback for Cameron

Andrew Mitchell has resigned as the government Chief Whip.

He asked to see the Prime Minister after he returned from the EU Summit in Brussels.

I spoke to Mr Mitchell while he was still at the prime minister's country retreat and he told me that he no longer felt he could be an effective chief whip responsible for the discipline of Conservative MPs.

He had reached this conclusion after spending four days back in the House of Commons following the party conferences.

During that time he was the focus of Ed Miliband's questioning of the prime minister at question time in the Commons and the subject of a debate amongst Tory backbenchers about whether he could stay.

In addition, his Deputy John Randall had a meeting with the prime minister in Number 10 after some critics of Mr Mitchell claimed that he would resign if his boss didn't.

This is a serious setback for David Cameron as he held onto Mr Mitchell instead of sacking him straight after his angry clash with a policeman on the gates of Downing Street.

The prime minister repeatedly insisted that his Chief Whip had apologised, the apology had been accepted and that was the end of the matter. It wasn't.

Mr Mitchell will not be doing interviews but will, he tells me, seek to deliver a personal resignation statement in the Commons early next week.