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Boris Johnson is political box office. He is one of Britain's most charismatic politicians. So his decision to support Brexit gives popular appeal to a Leave campaign that has lacked a talismanic figurehead.

And polls suggest that his is a voice that many will listen to as they make up their minds. So the mayor of London will transform the referendum campaign. But will he transform the result?

Until now Mr Johnson has won his votes as a jovial character in two regional elections. He is now testing his appeal on the national stage in a contest of historic importance. And that matters because Mr Johnson could be our prime minister one day.

By supporting the Leave campaign, Mr Johnson will win the support of many Tory members who in a few months or years will choose David Cameron's successor.

Mr Johnson this afternoon denied that this was his calculation. But many Tory MPs - on both sides - distrust his motives and suspect he is acting in his own as much as the national interest.

Only weeks ago Mr Johnson told a fellow MP that he was not a "natural outer". So can a man who has procrastinated so long over an issue he admits is finely balanced really campaign for Leave with the deafening eclat that he has promised?

Mr Johnson will bring vim, verve and vitality to the Leave campaign. But he has only one vote, just like the rest of us.