Europe’s press could not quite believe it either. “The Queen’s jester,” was the headline in France’s Libération as Boris Johnson prepared to enter No 10 Downing St. “He’ll start breaking his promises tomorrow,” warned Der Spiegel in Germany.

In a scathing portrait of the new prime minister, Le Monde said Johnson was “known for his eccentricity, his elastic positions, his narcissism and his lies”. His “insatiable need to be liked and phenomenal aplomb” meant anything was possible, the paper said. “Boris Johnson has a rendezvous with history – his lifelong dream.”

Libération said Johnson would “have to move fast to unveil his Brexit strategy” – something of a problem as “the possibilities open to him to keep his promise of leaving the EU by 31 October are extremely limited ... If the British government has changed, the obstacles remain the same.”

In its portrait of the former London mayor, the paper said Johnson was “finally about to imitate his idol, Winston Churchill, by taking his country’s destiny in hand at a time of existential crisis. But he has promised not blood, toil, tears and sweat, but a brilliant future – because the UK is ‘a great country’.”

Johnson “advances by the seat of his pants”, the paper said, “like a chameleon, adjusting his positions according to the moment, counting on his people skills and perhaps his luck. For long, his eccentricity and self-mockery – the essence of Englishness – saved him. That may no longer now be the case.”

In Germany, the Frankfurter Allgemeine Zeitung was equally brutal. “The clown who wanted to be king of the world” was its headline. Given Johnson’s well-known nonchalance and complete unpredictability, “Britain’s political drama is about to begin an act which will have its entire audience holding their breath.”

Der Spiegel said Johnson had been “dreaming of this for decades. But once he begins his new life as prime minister, he will have to stand in front of the door to No 10 and explain to his compatriots what he actually plans to do. And at that moment, he will begin breaking his promises.”

In recent weeks, the magazine said, Johnson’s “one-of-a-kind talent for talking a lot without really saying anything” had allowed him to “pledge all kinds of things to just about everyone. He cannot keep all of them.” His charisma “has got him far”, it added. “Now he has to show he has powers of leadership, not just of seduction.”

Die Welt said the hour of truth was fast approaching. “Johnson is about to enter No 10,” it said. “He must now prove he can master the country’s worst crisis since the second world war. It is not just the opposition that doubts he can.”

The Dutch daily NRC Handelsblad said the new prime minister would soon learn that “what he promised as a candidate cannot be achieved as prime minister. Over the past few years, Boris Johnson has in his speeches and columns claimed that he knew far better than Theresa May. Now he’s going to have to negotiate.”

The Irish Times despaired. “How far can Britain fall?” it asked in its editorial. “The Brexit debacle has already left the country bitterly divided, its parliament paralysed, its influence diminished, its reputation shattered. Now, a new nadir: Boris Johnson is set to become prime minister. That means things could still get a lot worse.”

The paper said Johnson was “a profoundly unserious man, wildly unsuited to high office … In normal times, his ineptitude would hold his country back. In these times of crisis, his character flaws could prove catastrophic.” And he has “no idea how to reconcile the need to avoid a hard border with an exit from the customs union”.

Denmark’s Politiken said that with Johnson in charge, “British politics could, incredibly, become even more unpredictable and chaotic”, while the centre-right Berlingske observed that the new prime minister “looks like a man who has slept in his car … and that’s his style”.

Spain’s El País said few prime ministers can have faced “such a complex and urgent challenge since the first day of their term”. But beyond the characteristic “enthusiasm and gung-ho spirit” that was on full display during Johnson’s campaign, “reality will be tough”, the paper warned.

“No candidate in the history of British politics has been more forgiven his many blunders and scandals,” it said. “Starting today, Johnson will find himself in a job in which all such incidents will carry real-world consequences.”