Boris Johnson is not a disaster waiting to happen. He is a disaster that has been repeatedly happening for years now, and the only change is that the disasters have got progressively bigger. He should have been sacked as foreign secretary months ago, when his blundering intervention in the case of Nazanin Zaghari-Ratcliffe – the charity worker jailed in Iran on spying charges, after what her family has always insisted was a simple visit home to see relatives – gave a hostile regime an excuse to prolong her ordeal.

Having survived that gaffe only because Theresa May was too weak to move him, Johnson should at least have had the grace to learn from it; to grasp that the Foreign Office is not the place for winging it or exaggerating to make a point; that even tiny deviations from the script carry grave consequences when dealing with intelligence-related matters or regimes such as Iran.

Johnson has a nerve accusing Jeremy Corbyn of undermining national security when he himself has handed Moscow a propaganda victory on a plate

But he wasn’t sacked and he didn’t learn, so here we are again. A slapdash interview the foreign secretary gave in Germany, in which he appeared to claim he’d been personally and categorically assured by “the guy” at Porton Down that Russia was behind the Salisbury poisoning, backfired on him this week when Porton Down’s chief executive, Gary Aitkenhead, explained that scientists could certainly identify the nerve agent involved, but that naming the culprit was above their pay grade. So much for the lessons supposedly learned from the run-up to war on Iraq about the incredibly difficult art of explaining intelligence-driven decisions to the public without compromising either the identity of sources or the accuracy of the material.

Anyone claiming that all this blows a hole in the idea of Russian guilt needs to cool their heels. It doesn’t change a government case that always relied on science to narrow the range of suspects, by identifying the means of poisoning, but on the intelligence services to complete the jigsaw. It was for them to advise on who might have not just the significant technical capacity required to deploy a Russian-manufactured toxin, but also the desire to kill enemies of the Russian state; the audacity to do it in a way bound to cause a crisis in Anglo-Russian relations, given its uncanny similarity to previous Russian operations; and the opportunities to get it in and out of the country. The case for this mysterious culprit being Russia was never definitive, but unless anyone produces strong evidence to the contrary, that’s by far the most likely explanation. That much hasn’t changed.

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What has changed is the credibility of the government in saying so, because the foreign secretary’s over-egging of the pudding – whether by carelessness or design, and in the circumstances both are inexcusable – gives Vladimir Putin a perfect excuse to cast doubt over anything and everything the British government says. Frankly, Johnson has a nerve accusing Jeremy Corbyn of undermining national security when he himself has handed Moscow a propaganda victory on a plate, dragged politically neutral Porton Down staff into an uncomfortably partisan political row, and needlessly undermined the work of the wider intelligence community.

But there is a broader problem here, one that awaits any future Labour government as much as this Conservative one, and which Johnson’s departure may be necessary but not sufficient to solve. That problem revolves around the management of doubt.

Politicians don’t like doubt, and neither do voters. Admitting to being less than 100% sure about anything – from the safety of a childhood vaccine to the cause of a rise in knife crime to the likely consequences of military action – is still seen in politics as a weakness. At best it allows critics to pounce, at worst it creates space for conspiracy theorists and malignant actors flourish.

Yet intelligence-led decisions invariably involve some degree of doubt. We can be certain about what put Sergei and Yulia Skripal in intensive care, and novichoks are so difficult to handle that Porton Down concluded that probably only a nation state could have deployed them. That “probably”, however, is where real certainty ends. The nature of intelligence is not only that it can’t show its workings in public for fear of jeopardising lives in the field, but that it’s invariably partial. There will always be secrets we can’t crack, people whose heads we can’t get inside, strong – even overwhelming – probabilities, but not cast-iron certainties. That’s why raw intelligence material comes peppered with caveats, because credibility depends on honestly acknowledging what you can’t prove or don’t know.

Facebook Twitter Pinterest Boris Johnson arriving at Downing Street. Photograph: Ben Stansall/AFP/Getty Images

But in politics, the opposite is true: authority flows from certainty. The difficulty comes when this political desire to sound authoritative meets the experts’ need for ambiguity. If politicians tell the truth about the holes in their argument, voters may not believe in whatever they’re advocating. Lie about it, however, and they certainly won’t.

Many of the parallels being drawn between this and the run-up to the Iraq war are wildly exaggerated, not least the idea that this could lead to war. But the common thread is that stripping out the ifs and buts in intelligence material never ends well. The single biggest problem with the infamous dodgy dossier wasn’t what went in, but the caveats left out, both in drafting and when the finished report was boiled down into newspaper headlines, which made its conclusions look so much more definitive than they were.

Downing Street appears to have partly absorbed this lesson, which is presumably why Theresa May said Russia was “highly likely” rather than certain to have mounted the attack, and the many other countries who back Britain’s stance talk of there being no plausible alternative explanation. These are phrases carefully chosen to sound authoritative, while leaving honest room for doubt. Or they were, until Johnson barged in.

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Those declaring Corbyn to have been triumphantly vindicated by all this are only half right. There is a clear contradiction between his theory that mafia gangs might have done it and Porton Down’s explanation of why the evidence points towards a nation state; it is not unreasonable to ask when Corbyn was made aware of that contradiction. But he is absolutely right that it should never be considered unpatriotic or inappropriate to ask searching questions, or require a government to explain itself. That is the duty of opposition, even when the nation is under direct attack. We are all allowed to have honest doubts.

The quid pro quo, however, is that governments of all shades should somehow be allowed to have honest doubts too; to admit gaps in their knowledge without instantly inviting derision, or the assumption that whatever you just read on Facebook is equally valid. With the right to question comes the responsibility to keep an open mind, to scrutinise all sides of the argument equally rather than hunting for reasons not to believe one of them. On the evidence of this week, sadly neither politicians, press nor public have got that balance right.

• Gaby Hinsliff is a Guardian columnist