The Conservative leader is understood to be holding open the option of withdrawing the Tory whip from the MP if he fails to say sorry for his remarks.

Mr Cameron made clear his displeasure when asked about the backbencher’s comments, made in an email to an angry voter, during a question and answer session following a speech in London.

In the message, written after he was accused of being a “trougher” for using his allowances to pay his own company £100,000, the MP compared the public outcry over the expenses scandal to the persecution of the Jews in Nazi Germany.