On Wednesday, a new prime minister will move into Downing Street. He will assume office at a time of burgeoning crisis. While the political system remains gridlocked over Brexit, the west’s deteriorating relations with Iran are fraught with the risk of global conflict. No prime minister has faced as challenging a set of circumstances since the Second World War. And the man who looks almost certain to be making his entrance through that famous door is Boris Johnson.

It’s hard to think of a senior Conservative MP less qualified to assume the premiership in such times. The clash with Iran highlights the wider dilemma Britain will face after Brexit. Isolated from our European allies, the danger is that we simply become a lesser satellite of American foreign policy, buffeted by global events and too weak to resist the pressure to do the bidding of Donald Trump.

And Johnson has a dreadful record as foreign secretary. He is best known for jeopardising the case of Nazanin Zaghari-Ratcliffe, the British-Iranian woman who has spent more than three years in a jail in Tehran, by erroneously stating that she had been “teaching people journalism”, later cited as proof by Iran that she was engaged in “propaganda against the regime”. It was a foolish misstep by someone who has demonstrated time and again that he has little capacity for paying attention to detail. His time at the Foreign Office was riddled with diplomatic gaffes, from reading a colonial-era Rudyard Kipling poem during a state visit to Myanmar to likening the challenge of the Irish border after Brexit to the London congestion charge, to saying the war-torn Libyan city of Sirte only had to “clear the dead bodies away” to become the next Dubai.

It’s not just the gaffes. Johnson has a casual disregard for the truth where it stands in the way of his career, or a good pun. Last week was no exception, when he brandished a smoked kipper from the Isle of Man at the final hustings of the Conservative leadership contest, and announced that its producer was “utterly furious” at the Brussels regulations that meant he now had to send his goods packed with ice. “We will bring the kippers back,” Johnson told the cheering crowd. Little matter that these rules were set by Britain, not the EU.

This is only the latest in a long line of occasions where Johnson has played fast and loose with the truth. Last month, he erroneously claimed that if Britain crashed out of the EU without a deal, we could avoid paying any tariffs on European imports. Rather than correct himself, he has doubled down – despite the fact that the independent Office for Budget Responsibility last week published its forecast that a no-deal Brexit would shrink the economy by 2% in 2020.

Boris Johnson and Jeremy Hunt: ‘Both said the Irish backstop was “dead” in a debate last Monday.’ Photograph: Rebecca Naden/Reuters

That should not come as a surprise: Johnson has form when it comes to wilfully misleading the public. His 2016 claim that leaving the EU would free up £350m a week for the NHS led to the head of Britain’s statistics watchdog chastising him for “a clear misuse of official statistics”. During the referendum campaign he claimed Turkey was on the verge of joining the EU, despite the fact its application had stalled, but this year claimed to have said nothing about Turkey during the campaign.

How can this man be trusted to lead Britain in the wake of a catastrophic no deal? For after six weeks of a dismal Tory leadership contest, that looks to be where Britain could be heading. In the run-up to the 2016 referendum, neither of the Leave campaigns mooted Britain crashing out of Europe without a deal – it would have been inconceivable. Yet as the Conservative party has drifted rightwards, captured by its anti-European fanatics in parliament, no deal has become normalised.

That process has accelerated in recent weeks as Tory leadership candidates have scrambled to outdo each other with Brexit extremism in an attempt to appeal to the 160,000 party members who will choose our next prime minister, the majority of whom are willing to countenance significant economic damage, the breakup of the union and the destruction of their own party to see Brexit happen. Both Johnson and his rival, Jeremy Hunt, said the Irish backstop was “dead” in a debate last Monday. If the next prime minister sticks by these words, it would vastly increase the risk of a no-deal Brexit.

Beyond Brexit, this leadership race has edged the Conservatives rightwards on domestic policy, too. For evidence of that, just look to Johnson and Hunt’s tax pledges. After a decade in which Conservative chancellors have loaded the burden of austerity on low-income families with children, all the while cutting taxes for businesses and the more affluent, both candidates would continue to prioritise reducing the tax burden for those with the broadest shoulders. That should be the clue as to who the next prime minister would expect to bear the pain of a no-deal recession: it will be the least affluent families and the poorest regions of the country.

The only remaining hope lies with parliament. We can take heart in the fact that the small number of remaining Tory moderates voted with the opposition last week to take action that would prevent a future prime minister proroguing parliament in the run-up to 31 October. Even if the only route to preventing no deal in the autumn is a no-confidence vote that triggers a general election, they must seize it. Otherwise the process of appeasement of the militant Tory Eurosceptics, which started with David Cameron and continued under Theresa May, will end up not only destroying the Conservative party, but taking the country down with it too.