Boris Johnson says he is desperate to get Brexit off the agenda for his own government, so that it can start applying blue cement to the bricks he turned blue in Labour's red wall – or throw money and popular policies at the midlands and northern seats he recently pinched from Labour.

In fact he tried to persuade me, in an interview during the election, that only saddos like me will be remotely interested in the details of the trade and security deal with the EU he courageously believes can be negotiated in a record-breaking 11 months.

For better or ill, he may be right. I am flabbergasted that many of the business leaders to whom I speak, who hated Brexit, have capitulated and are now Johnson groupies.

But for the opposition, Brexit should and must rear its head, at least till a new leader is announced on 4 April.

Because the Labour party's mishandling of how and whether the UK leaves the EU may represent the greatest fluffed opportunity of any opposition in history.

The open goal stretched from Humber to the Mersey, and Labour still missed.

Here are the relevant facts.

Labour fought the 2017 general election on a platform of delivering the result of the 2016 referendum, viz withdrawal from the EU.

It then saw a Tory party tearing itself apart over whether that Brexit should be softish - which was the position of the then prime minister, Theresa May, and not a million miles from Labour's own official position - or whether Brexit should be hard and "clean", the cause of the Brexit ultras of the European Research Group (ERG).

If Labour had voted for the kind of customs-union based Brexit that May was shaping, and forced it into law, the Conservative party would almost certainly have torn itself apart.

This is not ex-post speculation.

Given that Johnson now benefits from what the former Tory chancellor Philip Hammond calls a parliamentary dictatorship, it is easy to forget that less than a year ago the Brexit-induced destruction of the Conservative party was very much on the cards.

Tory MPs in the ERG were not bluffing when they said proper Brexit mattered much more than the survival of their party.

If Labour had called May's bluff and underwritten her Brexit - which would probably have kept the UK in the customs union forever thanks to that backstop that was detested by the ERG - Labour would have eviscerated the Tory party while appearing principled and statesmanlike.

In other words, Labour had the holy grail of politics in its grasp: the ability to trounce opponents while doing what many would have seen as the right thing.

Instead of which, Labour went through a charade of negotiating a Brexit compromise with May, while never being remotely serious about doing a deal (which is what those close to Jeremy Corbyn told me very explicitly at the time). Labour played Brexit as a game.

What is the party's harvest?

Johnson has united his party by purging it of those Tory MPs who wanted a soft Brexit, and it is now Labour tearing itself apart over its future direction.

And by promising "to get Brexit done" (sorry!!!!) Johnson won a near landslide and humiliated Labour, which today has fewer seats in the Commons than at any time for 85 years.

To add insult to Labour's humiliation, Johnson will deliver a Brexit that will be much closer to the hard break with the EU favoured by the ERG than that of May or Labour's 2017 election manifesto.

Or to put it another way, Labour's strategic failure on Brexit is arguably as catastrophic as it could possibly have been.

All of which surely means that the candidates to be Labour leader must say what - in retrospect - Labour should and could have done differently.

Because there is little hope of redemption for the party if it cannot do the work on how it fluffed the greatest opportunity in its history, namely the chance to devastate its great historic enemy.

This question is most serious for those candidates - Starmer, Long Bailey, Thornberry - who were in Corbyn's shadow cabinet and therefore have their hands deepest in the blood of an egregious opportunity cost.

But Lewis, Nandy and Phillips must also look back, to understand the lesson.

To be clear, it may be that the biggest mistake made by Corbyn's Labour was to campaign in that 2017 election on a platform of delivering Brexit, rather than at that early stage opting wholeheartedly for a referendum and remaining in the EU.

But once the manifesto was published, Labour's obligation in the British system was to do what the manifesto promised.

Maybe there is some kind of natural justice that it has been so punished for reneging on the manifesto.

But in the absence of owning up to the putative political crime, rehabilitation looks a distant prospect.

Robert Peston is ITV’s Political Editor. This article originally appeared on his ITV news blog