In The Usual Suspects, Kevin Spacey has a line for all Remainers laughing at the sight of Boris Johnson striding to power and then tripping over his shoelaces and falling flat on his face. He tells of a “gang of Hungarians that wanted their own mob. They realised that to be in power, you didn’t need guns or money or even numbers. You just needed the will to do what the other guy wouldn’t.” If they broke even gangsters’ lax conventions – not just kill a rival but his wife and kids – they would dominate.

Johnson is showing a ruthlessness no one has seen in British politics. “When are you fucking MPs going to realise we are leaving on 31 October? We are going to purge you,” the Mail reported Dominic Cummings screaming at Greg Clark. Cummings and Johnson duly purged not just the former business secretary but two former chancellors of the exchequer, Winston Churchill’s grandson and 18 other “fucking MPs” who refused to do as they were told.

You don’t give men like this the benefit of the doubt or pretend that nothing has changed. Harold Macmillan’s “Night of the Long Knives” in 1962 destroyed his reputation for calm confidence. But there’s no comparison. Macmillan merely sacked seven colleagues from his cabinet. He did not destroy them as Johnson and Cummings have done: strip them of the Conservative whip and end their careers by making it impossible for them to return to parliament.

We are in a new world and as opposition parties confronted with political gangsters from Trump’s America to Orbán’s Hungary have learned, you must believe them capable of anything and act accordingly. The British opposition should not think of trusting Johnson to “do the right thing”. It should ask how it can stop him and, once it has an answer, find the determination to act. Not trusting Johnson means there cannot be an election before late November, because parliament must be sitting in the days after Saturday 19 October when Johnson will be mandated by act of law to seek an article 50 extension from the EU.

The opposition should wait until it is sure he has no tricks to pull, which… means no election before 21 November

If you treat the Johnson administration as a crime gang, you will understand why. Parliament is dissolved for a general election and does not meet again for five weeks. What happens if Johnson takes the chance to refuse to ask for an extension to article 50? That, apparently, is his intention. He’d “rather be dead in a ditch” than obey the law, he said on Thursday. As the pressure built and he started to crack, he stammered on Friday: “I will not ex… I don’t want a delay.”

Constitutional lawyers and opposition MPs believe that, though he might have trashed parliament and the Conservative party, Johnson cannot trash the rule of law. He would be taken to court and found in contempt. Rather, that is what they assume, because there are no precedents.

The only case law comes from 1994 when the court of appeal ruled that the home secretary could be held in contempt after his officials unlawfully deported a Congolese asylum seeker. In truth, the case offers little help. The asylum seeker was deported by mistake – this is the Home Office we are talking about. It was an accident, a blunder that is not comparable with the supreme court ordering Johnson to go to Brussels to demand an extension or be confined in solitary in the nonces’ wing at Belmarsh to protect him from attacks from prisoners who do not appreciate his hard line on law and order, delightful though that prospect is.

Lawyers prepared to imagine such a future are certain that judges would want parliament to depose Johnson with a vote of no confidence and save them from jailing a prime minister. But parliament cannot intervene if it is not sitting.

Too fanciful? Try a second scenario. Constitutional lawyers purr with admiration when they contemplate the drafting of Hilary Benn and Alistair Burt’s bill forcing Johnson to seek an extension if his phantom negotiations fail. They are particularly struck by the beauty of subsection 3, which seems to cover all bases. If the EU offers an extension date other than 31 January 2020, Johnson will either have to accept it or return to the Commons to ask its opinion. It appears to be foolproof or Johnson-proof, which amounts to the same thing. But no one should be surprised if government and Vote Leave lawyers are busy looking for reasons to challenge it in court. Once again, parliament cannot plug loopholes if it is off on a five-week election campaign.

Alternatively, as many people are speculating this weekend, Johnson could resign and force Jeremy Corbyn to go to Brussels in his place. Who knows what he will do? I doubt he does. But if there is to be a new PM, would not MPs want to be in Westminster to have a say on the matter?

The historian Peter Hennessy authored the “good chap” theory of British government. Whereas other countries were governed by written constitutions, our politicians and civil servants ruled discreetly, because they respected unwritten conventions and never pushed their power too far. For three years, Brexit has drained the goodness from the system. But it has taken the chaps in the Johnson administration to empty it.

Meg Russell, of University College London’s Constitution Unit, and Jack Simson Caird, from the Bingham Centre for the Rule of Law, both emphasised the administration’s contempt for parliamentary democracy. Whatever scenarios they imagined, they agreed, as everyone should agree, that it is risky in the extreme for MPs to leave Johnson to his own devices.

The opposition should wait until it is sure he has no tricks to pull, which by my reckoning means no election before Thursday 21 November at the earliest. It should not trust him. It should not believe him. As the cop in The Usual Suspects bellows, in language Cummings would understand: “You get NO immunity from me, you piece of shit!”

• Nick Cohen is an Observer columnist