Those campaigning against the madness of our current predicament should not lose heart. This battle can still be won. In the past weeks, the mood on Brexit has shifted. Much of the public, through a somewhat paradoxical mix of boredom and alarm, have had enough. They want parliament to “get on with it” – no more delay and debate. They demand a decision. For some, even the calamity of no deal or the prospect of jeopardising the Good Friday agreement holds no force.

Never mind the detail. Just get it done.

This is helping the prime minister to win over Tory MPs, stoking the numbers of Labour MPs who, through old left anti-EU ideology or “the will of the people”, will vote with her; and putting those supporting alternatives, including a fresh referendum, on the defensive.

She is selling her deal based on what cannot be true – that if we agree it, the argument about Europe will end

The underlying sentiment is the completely understandable wish for closure, for an end to what has become a divisive, difficult and dangerous schism in the nation. But it rests on a gaping hole at the heart of Theresa May’s case. There can be no closure unless there is clarity about the future economic relationship with Europe. And on this point, the government has performed an extraordinary about-turn.

Remember when the prime minister used to say that we would know the future relationship with sufficient detail to make a judgment? When the Brexit secretary used to say that there was no question of a blind Brexit? When the phrase “meaningful vote” was coined precisely so that parliament, when it took a final decision on the deal, would know not only the terms of withdrawal but the shape of the future trading framework? When people excoriated the notion that we would leave without knowing where we were going?

What once was a vice is now a virtue. The political declaration in the withdrawal agreement is vague to the point of comedy. It leaves completely open whether we go in the direction of Norway, and soft Brexit, or Canada, and hard Brexit.

But now this is apparently an advantage. Tories who passionately want soft Brexit, and those who, equally passionately, want hard, cheerfully collaborate in arguing for a Brexit “without prejudice to the future relationship” and for a longer transition period.

Labour MPs who want Brexit with a permanent customs union happily vote with the prime minister, who has expressly rejected one.

It’s almost as if the difference between soft and hard were a mere bagatelle, a slight nuance, easily resolvable in a civilised discussion between friends.

The reality is that the difference between soft and hard is vast and measured in consequence for jobs, living standards, investment, indeed our economic future as a country.

It is the root of the entire Brexit campaign.

If we stay in the single market and customs union, then the huge interconnection of commercial arrangements that have grown up around 45 years of European membership will remain; but under the European rules.

If by contrast we pull out of them, then the economic disruption as business adjusts to losing the privileged access to a market whose rules govern 60% of our trade, will be large.

It is why the disagreement over Brexit is so intense.

It is why the negotiation with Europe was never a conventional negotiation; at core it was a choice – hard or soft. “Cake and eat it” was never achievable. No choice; no closure. Postpone the choice; and you postpone the closure.

May is selling her deal based on what cannot be true – that if we agree it, the argument about Europe will end. It won’t. It will just take new form. She herself knows this. It is why she tried to bring clarity with the now defunct Chequers proposal. It was heavily disguised clarity but even then, it provoked resignation and crisis. So she retreated into full ambiguity.

She has an alliance in parliament of three factions: Brexiteers, who hate the deal but think they can win the subsequent fight; those for whom the June 2016 result has become a divine instruction; and the quiet-lifers.

None of them believes her deal is good. No one believes it is good. Its sole selling point is that it ends the agony. But, without clarity as to the future relationship, it doesn’t.

Hard Brexiters say only acceptable way forward is to remove backstop Read more

Against May’s alliance should come a different one: those MPs, whatever their Brexit opinion, who recognise we need to know the destination of the train before we get on it and depart the station; especially when in time we will come to a fork in the track leading in diametrically opposite directions, with a driver who doesn’t know which we should take, several would-be drivers in the cab trying to wrest the controls and no way back to the station.

No amount of time will change the choice. Either we choose soft or hard, or decide to go back to the people. Any extension of article 50 should be mandated with a clear plan to make the choice, before we take the irrevocable step of leaving. MPs should explain frankly to the country that leaving without deciding will not bring closure but more chaos.

As every day passes, we are doing damage to Britain. We shouldn’t kid ourselves: economic confidence inside and outside the country is corroding; and our reputation as the nation of “common sense” is being severely undermined. The country gets this. So rightly it wants closure. Imagine its anger when, this time next year, we’re having the same debates but this time without the possibility of thinking again about the journey.

I am opposed to Brexit. But we need to end the argument. Only decisiveness, before leaving, will do it. A responsible government would facilitate it. A responsible opposition would advocate it. But if they don’t, parliament should insist upon it.

• Tony Blair is a former prime minister and founder and patron of the Tony Blair Institute for Global Change