P icture the squalid scene that confronted the cleaning staff on the morning after a night before of drunken delinquency by the denizens of Downing Street. Wine stains on walls. Pools of sick. Empties spilling out of bins. Mounds of party detritus on the floor. The heart of government, the place where you’d most hope for sobriety in the middle of a pandemic, turned into a vomit-splattered nightclub. The only heroes in Sue Gray’s investigation into Partygate are the security staff who suffered abuse when they tried to break up illegal gatherings and the cleaners who had to mop up.

Now try to picture scenes of all-night boozing, puking, punch-ups, vandalism and law-breaking at Number 10 under any other prime minister. You can’t. Nothing like this happened under any of Boris Johnson’s predecessors. The character of organisations is immensely influenced by the example set by the person at the top. When that person is Mr Johnson, you get a culture of selfish, arrogant, entitled, amoral, narcissistic rule-breaking that combines, in the true spirit of the Bullingdon Club, snobbery with yobbery.

It is not just the fabric of the building that has been trashed during his occupancy – it is the reputation of the high office that he has despoiled. This is understood by many more Tory MPs than the minority who have called for him to go. Ask them why they are not then exercising their power to remove him and some will tell you that this is because the Gray report “lacked a smoking gun”.

Seriously, guys? Just how many “smoking guns” do you need? Numerous parties were held in Downing Street during some of the deadliest waves of the pandemic when lockdown rules were at their most stringent. They were often instigated or attended by the most senior people in the building. We have a photograph of the prime minister, standing next to a table laden with bottles of wine and spirits, at one of the parties that he repeatedly swore to parliament never happened. I dare him to try his latest risible alibi, that it was part of his “leadership role” to join boozy leaving dos for departing staff, to the face of anyone who was forbidden from holding the hand of a dying loved one. Eighty-three people have admitted to breaching lockdown rules in Downing Street. In total, 126 fines have been imposed, making Number 10 the most Covid law-breaking address in the country.

Not only were they flouting the law, the Gray report supplies ample evidence that they knew they were. One official advised attendees at a Christmas party to leave via the back door to avoid being spotted by photographers. A special adviser warned colleagues to be careful not to be seen “walking around waving bottles of wine, etc” ahead of a gathering, because it was timed to occur immediately after a televised news conference at which the public was exhorted to abide by the Covid rules. Another Downing Street staffer sent an email referring to “your drinks which aren’t drinks”. One of the building’s spin doctors worries that the notorious “bring your own booze party” is “somewhat of a comms risk”. The organiser of that gathering, Martin Reynolds, the prime minister’s principal private secretary, later sends a WhatsApp message saying “we seem to have got away with it”.

There you have the ethos of Downing Street under the sordid stewardship of Mr Johnson. See what you can get away with. His life motto turned into the degenerate creed of Number 10.

The Gray report is also a searing indictment of the civil servants involved. Mr Reynolds, otherwise known as “Party Marty”, is reportedly being lined up as our next man in Riyadh. Sending him to teetotal Saudi Arabia, which takes a hard line on illegal drinking, would show that the Foreign Office has a sense of humour. Many others are wondering why he still has a career in the civil service. The same question is being asked of Simon Case, the cabinet secretary. That role has traditionally demanded the capacity to “speak truth to power”, to warn ministers, including the prime minister, when they are crossing lines. Yet either Mr Case was too feeble to challenge so much disgusting misconduct or he was complicit in the debauched culture that the Gray report rightly excoriates. The only reason that Mr Case and Mr Reynolds are still drawing taxpayer-funded salaries is surely this: they can’t be removed without making it look even more outrageous that Mr Johnson is still in his job.

However long he hangs on to it, we can already be clear about one of the defining legacies of his premiership. It is the dustbin of history for the “good chap theory of government”. The phrase was minted by Peter Hennessy, the eminent historian, to describe the belief that Britain could get by with unwritten conventions about how politicians should behave, rather than a firm set of rules, because our politics was populated with honourable characters who could be relied on to do the right thing. If that theory were ever true, it has been tested to destruction by the rogue who is still squatting at Number 10 despite being a law-breaker who has repeatedly issued falsehoods to parliament. We now need to adopt a “bad chap theory of government”, which presumes that some politicians will behave abominably unless they are prevented from doing so by robust laws that are vigorously enforced.

I haven’t the space to list all the things that will have to be done to disinfect our public life once the Johnson regime is gone. Today, I will highlight three especially essential reforms. The ministerial and civil service codes need stiffening and the policing of them has to be placed in independent hands. Ad hoc inquiries, set up to try to deflect pressure and conducted by civil servants, are not satisfactory.

As an official, Ms Gray could not pass judgment on whether the prime minister and the cabinet secretary, her bosses, are fit to continue in office. The independent adviser on ministerial interests, the post currently held by Christopher Geidt, is not fit for purpose either. He can only investigate code breaches with the permission of the prime minister, who can simply toss aside the adviser’s verdicts, as Mr Johnson did when the previous adviser found Priti Patel guilty of bullying. Evidence can be kept from the adviser, as Lord Geidt discovered when he tried to investigate Wallpapergate. The remedy is to implement the proposal of the committee on standards in public life when it made 34 sound recommendations to improve the integrity of government. We must have a genuinely independent invigilator of ministerial conduct with the power to initiate inquiries, demand the production of evidence and publish findings in full without interference by Number 10.

Money talks in politics. The Owen Paterson disgrace, the Greensill affair and the Covid contracts scandal demonstrate that the rules on influence-peddling and conflicts of interest are too puny and there is far too little transparency about who is lobbying government. There are guidelines about what business jobs can be taken up by politicians and civil servants after they leave government, but the watchdog is toothless. That invigilator needs to be armed with legal powers and meaningful sanctions against rule-breakers.

MPs have to reassert the fundamental principle that ministers who knowingly mislead parliament must resign. The Commons has only gone part of the way by referring Mr Johnson to the privileges committee. Even if that Tory-majority body finds him guilty of lying to parliament, there is no guarantee that he will not try to cling on.

Tougher laws and enforcement will help purge our politics of unethical behaviour. It is even more essential that there is a change in the culture so that the lodestar of parliamentary and ministerial life is not seeing what you can get away with, but probity. That has to be led from the top so it is obviously never going to happen while Mr Johnson is still there. He waited until MPs had left Westminster for recess before issuing a rewritten and diluted version of the ministerial code that removed from the foreword the previous injunction on members of the government to behave with honesty, integrity, transparency and accountability. Just when you think he can’t debase standards in public life any further, this shameless prime minister goes and proves you wrong.

You can wipe wine stains off walls and mop vomit from the carpet. It is our institutions of government that will need a deep clean once the party animal at Number 10 is finally taken out with the trash.

Andrew Rawnsley is Chief Political Commentator of the Observer