Theresa May has looked out of her depth since she became prime minister. She has blown her majority, achieved nothing beyond clinging to power, and remains in post only because of a paucity of alternatives in her divided party. So a disunited country has a terrifyingly weak leader as it confronts its biggest challenge since the second world war. On Friday she must try to restore her credibility, so shattered both at home and abroad, with a speech that finally proves she has a precise plan for Brexit.

When she stands up in Florence – a city that was once a leading financial centre – she has to find words to unite her feuding party, silence fanatics on the right, soothe the concerns of business and soften the view across Europe that her government still has little idea how to seal Brexit. She needs to shift from shallow slogans that shroud this debate and detail a clear vision for departure that embraces transitional arrangements and future trading relationships.

Her task has been made many times tougher by the attention-seeking antics of her foreign secretary. Boris Johnson should have been sacked by now. He has become like a Donald Trump armed with a thesaurus and some knowledge of the classics – a flashy showman with flexible political opinions who, for all his bombastic rhetoric, focuses only on making Boris great. But, despite collateral damage to colleagues, nation and party, this selfish man will conclude that his mission has been accomplished. He’s prepared the ground to resign if needed, amid angry talk of Brexit “betrayal”.

As May prepared for her latest speech of a lifetime, equally significant was a blizzard of tweets from Johnson’s ally, Dominic Cummings, campaign director of Vote Leave and widely seen as the strategic brain behind Brexit. These were scathing about “the shambles now unfolding”, which he blamed on the “historic unforgivable blunder” of triggering article 50 too fast and, before adequate preparation. He is right: Britain is in a sorry mess and part of the problem was entering negotiations too soon. Article 50 was designed with a two-year timeframe to deter departures.

But, just as with Johnson’s efforts to take back control of his career, Cummings’s intervention implies that everyone else is at fault rather than the architects of this chaos. Such is their arrogant certainty, they brook no compromise. Cummings savaged the more pragmatic Brexit secretary David Davis, cabinet secretary Jeremy Heywood and Philip Hammond, the chancellor striving to protect the economy from the most devastating Brexit impact. Even Whitehall’s human resources machine came under fire.

We are seeing arsonists flee a burning building, having set it ablaze. Nigel Farage also talks about “the great Brexit betrayal”. These charlatans will claim it was all ruined by fainthearts. Meanwhile, the likes of Andrea Leadsom, the leader of the House of Commons who nearly ended up as Tory leader, and Labour’s Kate Hoey lash out at journalists who dare ask questions on withdrawal, making sinister accusations of unpatriotic behaviour to shut down debate. Prepare for more of this self-serving guff as the heat intensifies.

Cummings approvingly referenced Otto von Bismarck on revolutions. I have reported on several revolutions and seen a repeated trajectory in which ideals end up engulfed by dark forces. Certainly Britain today feels a far less comfortable and confident nation after last year’s divisive ballot, for all that bravura talk of a glorious new age unleashed by Brexit. Even Johnson has, reportedly, bemoaned losing friends over the referendum.

The more apposite Bismarck quote would have been the Iron Chancellor’s remark that people never lie as much as during a war or before a vote. We have just seen the shameless foreign secretary revive the risible claim that Brexit will give the NHS a £350m-a-week boost, ignoring official rebuke. “Why does he need to resurrect this?”, I was asked by one leave minister. “It was not true at the time and it’s not true now.” The answer is simple: it resonated more than any other issue, even immigration, in last year’s depressing debate.

These people are playing games with other people’s prosperity. They won the referendum by exploiting mistrust for politicians, and their legacy will be a weakened economy that makes it harder to address inequalities or build houses. They are also corroding their tortured party’s base in business and among younger voters, and probably handing the next election on a plate to Jeremy Corbyn, unfettered by Brussels.

The challenge facing an impotent prime minister is immense. May must choose whether to side with people such as Johnson, who preach nationalism yet show conceited contempt for fellow citizens and colleagues, or find a path through the morass they created that lessens the damage by retaining fluid and open relationships with our allies. Cummings has come clean and admitted Brexit is leading to an “unnecessary shambles”. Yet as the rats desert their sinking ship, they absolve themselves of any blame for the debacle.

• Ian Birrell is a former speechwriter for David Cameron