In a few months, Britons will be asked to decide whether their country should stay in the European Union or go it alone.

It's a question that cuts across traditional party lines. On one side of the debate stands Prime Minister David Cameron, who has staked his entire career on trying to persuade British voters to choose to stay in the European Union.

On the other, one of the more flamboyant figures in British politics: Mayor of London Boris Johnson. To many people’s surprise, ‘BoJo’ this week officially announced his intention to campaign against Cameron, and support an exit from the EU.

A long-rumored rivalry between the two former schoolmates has broken into the open. The two men have a remarkable shared history: Both went to the same exclusive school at the same time, and later attended the same university. Both are leading members of Britain’s governing Conservative Party.

But that shared background disguises a long running rivalry, according Sonia Purnell, author of "Just Boris: A Tale of Blond Ambition." For Johnson, replacing Cameron as prime minister is an ambition, she says. “Every fiber of his being desires that job. Every step in his life to date has been towards Downing Street. There’s no doubt about that. But he’s done it with charm. ”

Johnson's sometimes comical persona and public image has alwasy been key to his success. Purnell says this is more carefully stage managed than it appears. “He tries to defuse this idea of being ruthlessly ambitious by joking, by pretending to be a bit shambolic. [For example], his hair is always standing up on end — naturally it is quite neat hair, but he messes it up, because that denotes too much ambition."

Personal animosity between Cameron and Johnson appears to have increased since Johnson declared his position this week.

On Tuesday, Johnson was heard to mutter "rubbish, rubbish" in parliament as Cameron made pointed remarks about the personal ambition of those who support an exit.

For his part, Cameron was more guarded. He told a referendum campaign event: “I have huge respect for Boris. ... He’s a great friend of mine ... but on this issue I think he’s got it wrong."