They were schoolmates at Eton, members of the exclusive male-only Bullingdon Club at Oxford University, and campaigned for each other when they made it to Prime Minister and Mayor of London respectively.
Key points:
* Boris Johnson described David Cameron's argument about how leaving the EU could jeopardise security as "demented"
* At the premiere of Brexit: The Movie, Euroskeptic Tory MPs openly mocked the Prime Minister
* Mr Johnson among about 130 Conservative MPs who say they are in favour of EU exit
But the Brexit campaign has placed strains on whatever friendship David Cameron and Boris Johnson once had.
When Britain decides about its future in the European Union next month, only one of the pair might have a political future.
The battle between these two Conservative Party heavyweights began in February when Mr Johnson announced he was campaigning for Britain to leave the EU.
"He gave David Cameron nine minutes' notice," said Sonia Purnell, a former Whitehall correspondent and author of the unauthorised biography, Just Boris.
"He sent him a text saying what he was going to do, I don't think that shows the hugest amount of respect."
The referendum campaign has become increasingly antagonistic.
When Mr Cameron raised the point that leaving the EU could jeopardise security on the continent, the former mayor of London was scathing in his response.
He described Mr Cameron's argument as "demented".
As they walked up the red carpet at Leicester Square for the premiere of Brexit: The Movie, Euroskeptic Tory MPs openly mocked their Prime Minister.
"What the Prime Minister has been saying about us going to war is demented," conservative politician Jacob Rees-Mogg told the ABC.
Former major Government Minister John Redwood said: "I think the Prime Minister has done my side to leave the EU a great service. I think self-parody is a new art."
UKIP leader 'enjoying' watching Conservative Party tensions
Photo: Nigel Farage is enjoying watching the Conservative Party tear into each other. (AFP Photo: Ben Stansall)
Leader of the UK Independence Party, Nigel Farage, is enjoying watching the Conservative Party tear into each other in public.
"It's great fun, it's great fun," he told the ABC, as he walked up the red carpet in a suit he declared "was English".
Asked what it would mean for Mr Cameron's future if Britons voted to leave the EU, Mr Farage, replied: "Toast. Toast. Gone within days."
Outside the House of Commons, and away from the showbiz hype of Leicester Square's red carpet, Alberto Costa said the debate would not harm the party.
A Scottish Tory of Italian descent who holds the English seat of South Leicestershire, Mr Costa is in favour of remaining in the EU and welcomes the debate.
"I don't think there is any disunity here," he said. "We are politicians. We agree on a whole range of issues as a Conservative Party. We are one broad church and within that church there are differing views."
However Mr Cameron acknowledged the dangers of the Brexit debate for his party.
While he has agreed to a television debate with UKIP leader Mr Farage, he has ruled out debating any of his conservative colleagues, avoiding what his office calls a "blue on blue fight".
Mr Johnson is one of about 130 Conservative MPs who say they are in favour of exiting the EU.
Questions around former mayor's intentions
But a lingering question remains. Does the former mayor of London believe in what he is campaigning for? Or does he see it as his chance of succeeding Mr Cameron as Prime Minister?
Ms Purnell, his biographer, who worked closely with him when he was a journalist with the Daily Telegraph, thinks his position on the EU is about ambition rather than principle.
"Until just before he announced he was going to back Brexit, he was telling people, 'I'm no outer'," she said.
"I've known him for many years since the 1990s. I used to work with him in Brussels."
He would be writing monumentally critical copy on the EU, but at the same time in private he would be really sympathetic and defensive of it.
"I think really he [is] not an outer, but he saw an opportunity for himself," she said.
The referendum on June 23 is shaping up as more than just a vote about Britain's future in the EU.
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