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The Daily Express has learnt that the charismatic London Mayor, who steps down in May, held informal talks with a senior Conservative backbenchers before Christmas over the role.

But, in what many will see as a setback to the whole campaign, he made it clear he did not want to lead the Brexit drive.

It is understood he said: “The trouble is, I am not an ‘outer’.” Mr Johnson was widely considered a Eurosceptic.

In November, when asked about joining the campaign, he only said he was “waiting” for Prime Minister David Cameron’s renegotiation.

But he signalled that he could lead the campaign to free Britain from Brussels by adding: “There is an attractive and alternative future [to staying in], we should be thinking about it.”