Prime Minister David Cameron has won the battle with his European partners, negotiating concessions for the UK. But he could lose the war on the Brexit at home - all because of London Mayor Boris Johnson.
The start of the week saw a return to business as usual in Brussels, with EU Finance Commissioner Pierre Moscovici announcing to the press that the EU doesn't have a plan B in case the UK votes to leave the European Union in June.
"The day we start talking about a plan B is the day we no longer believe in our plan A. I have just one plan: The United Kingdom in a united Europe," he said.
EU officials also made it clear that Brussels will not get involved in the upcoming UK referendum campaign. "I don't see the Commission having a role in a campaign that is for the British people and the British people alone," said Commission spokesman Margaritis Schinas.
After negotiations were concluded on Friday, German Chancellor Angela Merkel stressed that, backed by the deal struck with EU leaders, Cameron would be able to campaign for Britain to remain in the EU. A statement that could be translated to: "I, Angela, have done everything possible - the rest is up to you, Dave!"
Cameron, appearing on the BBC's flagship political program "The Andrew Marr Show" on Sunday morning, couldn't wait to show off his newly developed affection for Europe. And according to one poll, approval of EU membership had increased by 15 percent among Britons in the days following Cameron's success in Brussels.
Temporarily, everything seemed to be going rather smoothly for the prime minister, even though it was clear early on that several of his cabinet ministers would not join him for his "Stay" campaign, a decision that threatens to tear his Conservative Party apart. But that was before the blonde bombshell.
Boris Johnson: master of the game with marked cards
London Mayor Boris Johnson (above left) cultivates an image that's highly artificial, and yet, amazingly, comes across as natural. Boris, as he's known by the press and Londoners, loves to insert Latin phrases into his speeches - and despite this penchant, every London taxi driver would join him for a drink at the pub.
His suits don't fit properly, and they're always crumpled. His love life has been hit by allegations of an extra-marital affair. He is neither slim nor fit; yet he likes to appear in public in cycling gear. When it was revealed that Johnson had ostentatiously cycled to his office at City Hall, trailed by a staff car containing his briefcase, the nation laughed. For anyone else, this kind of deception would have been the end of a political career.
Johnson has crafted his image to allow Britons to identify with him. At the same time, he keeps his distance. He pretends to be a man of the people, while simultaneously showing that he isn't one. He stumbles around, flubs his answers and always looks as if he just climbed out of bed.
But behind that facade lurks a burning ambition. In a book about the childhood days of the Johnson clan, his sister Rachel wrote that Johnson has always had his sights set on one political prize: the role of prime minister.
Cameron and Johnson come from similar British upper class homes, have a similar education and have known each other since boyhood. And yet, they couldn't be more different.
Whereas Cameron's posh boy image is barely hidden by his urbane, polished exterior, Johnson has completely redefined his upper class upbringing. That's why he was somewhat appalled when, several years back, some photos from his youth were leaked to the public, showing Cameron, Johnson and current British finance chief George Osborne as young men at Oxford University. Dressed up in tuxedos, they're seen posing as members of the infamous and exclusive Bullingdon Club, a society of upper class toffs who spent their university years, financed by their respective fathers, drinking and rampaging.
The two men have known each other since their days at the elite boarding school Eton. Johnson was a star and head boy, whereas Cameron performed distinctly better at university. Cameron went on to become the leader of the Conservative Party while Johnson got himself involved in a number of scandals, which seriously called his eligibility for higher political office into question.
But Britons took to his blend of loose tongue and middle-class intellectualism, and eventually party leader Cameron had to ask him to run for mayor of London. In the traditionally left-wing city, the irreverent Johnson was the only one who could take the post away from the Labour party. He amuses people, and could be a professional stand-up comedian - and these talents have made him into the most dangerous weapon of the "Out" campaign.
All gloves are off
Over the weekend, Johnson showed off his broken heart, his indecision and his political agony, only committing himself late Sunday afternoon to the "Out" campaign. "There is only one way to get the change we want - vote to leave the EU," he wrote in a column for the Monday edition of the "Telegraph" newspaper.
Johnson claims that Britain could, after saying 'no' to the EU, get a new round of negotiations in Brussels and achieve even greater concessions. This tactic, however, had already been clearly rejected by some EU members at last week's summit.
It's believed that Johnson informed Cameron about his decision via text message. Now, all gloves are off. The prime minister knows that Johnson aims to take over as head of the Conservative Party, which would amount to toppling Cameron.
Johnson's popularity, his ability to say what people want to hear and his knack of disguising prejudice with high-flying rhetoric has made him the most dangerous opponent of the pro-EU campaign. And the slogan "Boris for Brexit" can't be beat for its alliterative simplicity.