Johnson has the most to gain - and lose
It was David Cameron's biggest gamble and he was convinced he would win. Now, the British leader's closest rival - in the EU referendum, in the Conservative Party and in the country - is the frontrunner to take his job.
Britain's decision to leave the European Union, a victory for the populist cause, may be a defining moment for Britain and the European project. The one man to ascend is Boris Johnson, the former London mayor and schoolmate of Cameron's who became the household name of the 'Out' campaign, even as the prime minister battled for 'In'.
With Cameron's announcement that he will resign after the British public voted against him, Johnson is now the strongest candidate to replace him.
But the moment of victory is fraught with challenges - first he will have to convince Conservative lawmakers to back him and then persuade the party's largely eurosceptic wider membership, who may challenge what his critics call a record of changing his views to suit his audience.
For Cameron, the turning point came in February when Johnson, a politician who has built a broad appeal beyond the ruling Conservative Party, threw his support behind the 'Out' campaign in what several sources say was a calculated move to boost his chances of replacing the prime minister.
Aides to the prime minister said Cameron believed he could have easily won the fight to keep Britain in the EU if he had had the support of Johnson, who used his position as mayor of London between 2008 and 2016 to broaden his appeal beyond the Conservative Party by supporting gay rights and immigration.
The aides, and Cameron himself, had campaigned for Johnson's support, but while the prime minister and the former mayor had "always got on pretty well", their's was essentially a political relationship, a friend said.
"They had a relationship when they could afford to be quite ballsy with each other because they were close," the friend said, pointing to their shared background - both were educated at the elite Eton school and knew each other at Oxford University. Like almost every politician, Johnson is ambitious, the friend said.
And if his backing of the 'Leave' campaign was not entirely unexpected, it was frustrating, according to a former Cameron aide.
That frustration boiled over in parliament a day after Johnson declared his hand and criticised the prime minister's new deal with the EU of a "special status" for Britain that Cameron thought should bolster his case for remaining. After rubbishing a suggestion made by Johnson and others in the 'Out' campaign that a vote to leave could prompt a deeper renegotiation of Britain's ties with the block, Cameron said: "I am not standing for re-election. I have no other agenda than what is best for our country."
His aides deny it was an attack on Johnson, but for dozens of eurosceptic Conservatives and others who were undecided over which way to turn, it seemed to contradict Cameron's pledge to be respectful of those who disagreed with him.
Andrew Bridgen, a senior Conservative lawmaker who campaigned for Britain to leave, described the exchange as akin to a "pub brawl" when "you hit the biggest, most dangerous-looking bloke as hard as you can and hope he doesn't get back up again".
But with his party's right-wing in the ascendance, a new leader is expected to come from the eurosceptic camp, potentially putting paid to secure his legacy as a "moderniser" of the party and his much hoped for succession of George Osborne, the finance minister.
Johnson, whose persona as a 'buffoon' can charm and repel in equal measure, is at the front of the pack. But his gamble - abandoning his support of immigration to win more support from the Conservative Party which will elect its next leader, and new prime minister - may undermine his wider popularity.
There was evidence of that difficult road yesterday morning. As Johnson left his north London residence to make his first statement since the referendum result, he was booed and jeered by Londoners who may have once supported him.