What would it take to make those who voted for Brexit change their minds? That may turn out to be the dominant question of British politics over the coming years, as the reality of the British exit from the EU comes ever closer. Even the most diehard remainers concede that the only way it can be halted is if British public opinion has a change of heart, collectively repudiating the verdict it expressed in the referendum of 2016.

That question might seem irrelevant when put to the conviction Brexiteers who led the leave campaign: surely they would be the last people to admit to a change of heart. And yet, now there is a suggestion that even the mastermind of the leave campaign is having his doubts.

It came in a Twitter exchange between Dominic Cummings, who as director of Vote Leave was the man behind the £350m-for-the-NHS slogan on the side of the bus, and the legal commentator David Allen Green. Cummings wrote that he feared that unless pro-leave MPs asserted themselves, forcing management changes on both Downing Street and David Davis’s Brexit department, the Brexit talks with the EU would be a “guaranteed debacle.” Green then asked Cummings, “Is there anything which could now happen (or not happen) which would make you now wish leave had not won the referendum result?” The arch-Brexiteer’s reply was swift and arresting: “Lots! I said before REF was dumb idea, other things shdve been tried 1st. In some possible branches of the future leaving will be an error.”

odysseanproject (@odysseanproject) Lots! I said before REF was dumb idea, other things shdve been tried 1st. In some possible branches of the future leaving will be an error

Now, to be fair, Cummings was not saying that leaving the EU is bound to be a disaster: rather, that failure is just one possible future. (I think he was using the word “branches” in the sense of a decision tree, which envisages a series of possible choices and their consequences.) Earlier in the exchange, he explained his view that Brexit alone, while necessary, was not sufficient. For leaving the EU to work, there would also have to be wholesale reform of Whitehall, British education, science and productivity.

Facebook Twitter Pinterest Dominic Cummings Photograph: David Levenson/Getty Images

But even in that context, it’s still a striking statement, given who’s making it. The man who led the drive to pull Britain out of the EU is admitting that it may well not work. Indeed, given how unlikely it is that the entire British system of governance can be transformed in the way Cummings says is necessary, it’s fair to say that, on Cummings’ own logic, failure is more probable than success.

You can be cynical about this, believing that, after the £350m deception, no one should listen to a word Cummings says. Or that he is simply preparing the ground for a disreputable, if familiar, political manoeuvre. When Brexit goes horribly wrong, he wants to be able to say, “That’s because you didn’t do it right. If only you’d listened to me, and done my version of leave, everything would have been fine.”

You might also feel anger at the mildness of Cummings’ language. “Error” doesn’t quite capture it. Better would have been a tweet that read, “In some possible branches of the future leaving will be revealed as a national calamity, an act of self-harm that was utterly needless – and in which I played a central and shamelessly dishonest role.” Perhaps he couldn’t get that into 140 characters.

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Still, that hardly detracts from the significance of what Cummings has conceded. He’d already described Brexit as “the hardest job since beating Nazis” and argued that it was being handled incompetently. Now we know he thinks that Brexit talks are a “guaranteed debacle” and that, absent a total transformation of the British state and economy, Brexit itself will prove to be “an error.” That sounds a lot like a man who, 12 months on, is beginning to realise last year’s referendum went the wrong way.

There have been other, similar straws in the wind. There was the leave-voting owner of a Surrey fruit farm who told the Today programme he now feared his business – entirely reliant on migrant labour – would collapse in 2019. And a widely shared letter to the FT from a leave voter who wrote that the last year had made him see that the cost of exit would be “extremely painful” and that now “I unashamedly regret the decision.” But clearly Cummings’ intervention is the most significant.

There will be more in this vein, as the consequences of Brexit become more real. As people see the cost, and the meagre gains, of this massive upheaval, there will be more people who admit that they got it wrong. The question is whether that collective act of buyer’s remorse comes in time – or too late.