Boris Johnson is the dog that caught the airplane. He has broken every rule – from lying to hiding from questions in a fridge, from illegally proroguing parliament to slyly dispatching his two predecessors – to attain his childhood dream of becoming “world king”. Now he has to decide what to do with the prize. I suspect that, like the dog, he hasn’t got a clue. Johnson is an untethered balloon full of hot air which could float off in any direction. He is completely at the mercy of his advisers, particularly Dominic Cummings who, unlike his boss, has an agenda.

As I know from my experience in May 1997, the beginning of a new government following an electoral landslide is the moment when you have the political space to do anything you want. Tony Blair managed to get rid of the twice weekly sessions of Prime Minister’s Questions before anyone else had yet woken up from the aftermath of the election. The opposition is flat on its back and your own party is so grateful to you for winning that it is in a warm fuzzy glow and will not question anything you do. In the hands of the wrong people this is a very dangerous period for the country and its institutions.

Cummings has set out his agenda: taking revenge on the BBC by removing the licence fee, undermining the courts, turning Ministry of Defence procurement upside down, revolutionising the structure of government and upending the civil service, which he feels disrespected him when he was a special adviser in the Department for Education.

Some of these may be perfectly good ideas but, given Cummings’s track record over Brexit, allowing him to implement them without any scrutiny is beyond risky. We know Cummings has contempt for the conventions that make up our constitution. The damage he could do within a few months of unconstrained rampaging around Whitehall would be very difficult to repair later.

This is also a period when the country faces huge decisions about its future. Brexit is far from done and it is only now that we have to make the real decisions about our future relations with the EU and the rest of the world.

We face a choice between a close relationship where there are zero trade tariffs and zero quotas as long as we agree to a level playing field with Europe; or a more distant Canada-style free trade agreement, in which case we will not escape tariffs and quotas. The former could be negotiated by the end of 2020 but the latter could not – the Canada Free Trade Agreement was approximately 2,000 pages long and took seven years to negotiate.

By the end of the year we will be back in the Groundhog Day scenario of wondering whether or not we will leave the EU without a deal. The fury of Johnson’s new supporters will be unbounded when they discover that far from being “done” as they were promised, we are going to have to watch endless repeats of the Brexit drama for the next several years.

An even more existential challenge to the United Kingdom will be the calls for a united Ireland and an independent Scotland. Having erected a customs border between Northern Ireland and the rest of the UK, Johnson has put the province into an economic united Ireland. Coupled with demographic change this is likely to increase demand for a politically united Ireland too.

The British government promised in the Good Friday Agreement in 1998 to hold a border poll when a majority is in favour of leaving the UK. In Scotland, Johnson has said he will prevent a further referendum on independence even though support for leaving approaches 50 per cent.

He is perfectly within his rights to do so in legal terms. But the consequence of denying self-determination to the people of Scotland will be to drive support for independence still higher, resulting in the same mess as Spain has with Catalonia. It is in any case untenable to agree to a referendum in Ireland, as we are required to by an international agreement, but to refuse one in Scotland. After all, British rule in both places depends on the consent of the people who live there.

What is so dangerous is that all of this is going to happen when we do not have an effective opposition. We have survived bad and even mad governments in the past but only because we had a strong opposition that could hold them to account and prevent their worst excesses.

To be worthwhile, the opposition needs to be both a credible alternative government and capable of forensically challenging what the government is trying to do. In those terms we have not had an effective opposition for four years. But to continue in that position for another four months with Jeremy Corbyn facing Johnson at PMQs – just so Corbyn can self-indulgently preside over an inquiry into why he failed and help fix the selection of a Corbynista as his successor – is an abnegation of duty to the country.

This is a problem not just of the timing of Corbyn’s departure but also of who succeeds him. This election was a comprehensive rejection of Corbyn and Corbynism, and to replace the man but leave the politics the same would not solve the problem.

The Labour Party now has a final chance to break out of the extreme left cult and choose a serious leader who can hold Johnson to account and credibly look as if she can win an election. If Labour flunks the test again then there has to be an alternative. It is far too dangerous to leave Johnson without a competent opposition. A new progressive political movement will have to take its place as the principal resistance to Johnson and Cummings.

Jonathan Powell was chief of staff to Tony Blair from 1995 to 2007