In the next 24 hours we are about to find out exactly how powerless Theresa May’s Brexit strategy has rendered our country. When she enters the European Council meeting in Brussels on 10 April she will be confronted with one of three potential outcomes.

The one she wants is the postponement of Britain’s leaving date until 30 June (from 12 April), making the UK’s participation in next month’s European elections obligatory but pointless. The one she thinks she’s going to get is an open-ended extension until at least the end of the year. This would destroy May’s administration, probably split the Conservative Party and – crucially – leave time for either a general election or a second referendum.

But there’s a third outcome which, until now, people assumed was a bluff: that president Macron of France, supported by European liberal leader Guy Verhofstadt, decides to teach not only the Brits but everybody else a lesson. Using the veto power of one or more states, this alliance could force May to confront the possibility of a no-deal Brexit, declaring immediate emergency measures to soften the crunch, which would happen on Friday night, just as the pubs are closing.

As they decide what to do, the 27 heads of government will have an eye not primarily on May, but on the potential response of British politics and civil society. Hence what we do and say – in the streets, on talk radio, in the op-ed pages and on Twitter – actually matters.

With this in mind, let me run through what I think Labour – a significant force in civil society, with half a million members – should do in response to each scenario, and which one we should support.

First, in response to the nuclear option of no-deal, we need to plead – and I mean plead – with the EU not to do it. It would trigger major trade disruption, massive financial uncertainty and tank economic growth in the space of a few weeks. Businesses would go under – and in an atmosphere of chaos, the xenophobic nationalism being stoked by the tabloids and the Tory right would run riot.

If May comes home from Brussels with a clear no-deal ultimatum, parliament would have less than 48 hours to unilaterally revoke Article 50. That means cancelling Brexit. As Labour’s shadow business secretary Rebecca Long-Bailey intimated on The Andrew Marr Show on Sunday, the party would have to short-circuit its own internal divisions over a second referendum or a Norway-style deal and simply stop Brexit.

The aftermath would see May immediately replaced, a Tory civil war, and outrage among the chaos-merchants of the far right – but Labour would go into the next political period with clean hands and a reputation for decisiveness.

By my calculation, while there may be 25 Labour MPs who want to “get Brexit over with”, fewer than six would actively embrace no-deal.

Second, what if May comes back with a short, final extension to 30 June? It would make a mockery of participation in the European parliamentary election, unless Labour made clear that its aim was a further extension beyond that. Any idea that we can sign an amended political declaration, at the point where May is due to be replaced by Boris Johnson, who could then rip it up, should be rejected.

Finally, what if the EU offers a flexible extension, ending for certain in nine months’ time or longer, removing the UK’s vote over budgets until matters are resolved?

For Labour – indeed for British democracy and the European project itself – this is the best option. Because time will allow two sets of people to have a say in the crisis who have been up to now excluded: the wider electorate, and Labour’s active membership.

If we get a long extension, I urge all internationalists and progressives inside Labour and beyond to adopt this three-point plan.

1. Campaign in the European elections for Remain, reform and a rewrite of the Lisbon Treaty.

2. Commit the Labour Party, via the conference resolution and delegation process and the election of the National Executive Committee (NEC), to a Remain-reform strategy.

3. Commit to using the extension to stage either a referendum or a snap general election, making clear that in either case Labour would campaign to stay in the EU in order to fight its neoliberal strictures.

There will be numerous people who disagree and I want to address their arguments here.

There is one strand, mainly constituting the old Labour right, which wants to deliver Brexit because it cannot face explaining to Leave-supporting voters that no-deal is a disaster and that the parliamentary process has failed.

Others support the idea of remaining, but fear the social divisions that will be stirred if there’s a second referendum. They have a justifiable fear that Labour could lose some seats in heavily Leave-supporting areas.

Finally, there are the Stalinists and Blue Labour supporters who actually want Brexit in order to “break up the global system”, even to the extent – as with Labour peer Maurice Glasman and the Communist Party leadership – of calling for no-deal.

A long extension changes the world for all three tendencies.

First, it is guaranteed to remove May, and either install a Johnson/Amber Rudd leadership or split the Tories in two. With a strong, principled and united shadow cabinet, Labour could hammer home the lesson to its own voters who supported Brexit: they’ve been betrayed.

Against a Tory party that has swung rightwards, and amid rising violent rhetoric from the far right, Labour’s task would be to construct a political alliance with the other progressive parties to keep Britain either in, or as close as possible to Europe. Whether it’s Remain or Norway, we end up at war with the English nationalist right. No amount of trimming and hedging gets you around that.

A long extension is the route to what all wings of the Labour party (except the depleted Blairites) actually want: a general election. To the people obsessively counterposing an election to a second vote, well here’s your chance.

But the biggest advantage of an extension is that it will allow Labour’s membership finally to dispense with the nonsense being propagated by some party officials and a minority of shadow ministers. We are not, and have never been, a “Leave party”. Leaving was forced on us by a referendum result that – as has been demonstrated – cannot be enacted by the present parliament. Just as the next parliament can’t be bound by this one, neither can a Labour government be bound by the referendum of 2016.

There are two scenarios arising from a snap autumn general election: a Labour majority government and a Labour-led de facto coalition with the SNP/Plaid Cymru/Greens. If the Tories split we get the former; if not, probably the latter.

What most people have failed to realise is that, in both scenarios, whatever Labour puts in its manifesto, Brexit is finished. If Labour had a majority of 25 it would only take exactly that number to refuse to vote for a Labour-designed Brexit deal for it to fall. Corbyn would be left reliant on Tory MPs and the DUP to push through a policy his members reject. It would be not a good look.

So better, instead, to begin the process now of sending something like the following resolution to Labour conference, backed by delegations prepared to support it and an NEC prepared to enforce it.

“This conference notes the collapse of Theresa May’s Brexit strategy. It reaffirms Labour’s 2016 position that Britain should remain in the EU, while seeking reforms that remove barriers to state aid, nationalisation and enhanced workers’ rights. It commits a Labour government to initiate changes to the Lisbon Treaty in line with the above. It commits an incoming Labour government to a massive and immediate programme of investment into deindustrialised areas. It mandates the party leadership to fight for this policy at the next general election.”

Unlike last year, I am not in favour of allowing this position to be composited with its opposite. I would rather hear all the arguments for Lexit put honestly and passionately from the conference floor. That done, we need to get on and represent the progressive part of Britain, and stop triangulating with the ideas of our bitterest opponents.

The Lexiteer brigade will claim such a strategy will damage Labour’s ability to win seats in Leave-supporting areas. But the Leave movement is fragmenting: with a strong economic offer, and patient work on the doorstep, we can win back those not attracted to the no-deal fantasy. The key to doing this is radicalism: the Hansard survey, which found rising numbers of British voters craving a “strong leader who breaks the rules” shows what we are up against.

Either we, the labour movement, provide a strong, clear vision and lead a revolt against poverty and austerity, or the right will lead a revolt against an open society and the rule of law.

And don’t wait for conference. As the European Council mulls its response to May, and asks its diplomats “what is Labour thinking?”, thousands of party activists could have an effect over the next 72 hours by signalling our intent to fight, one more time, for Britain to be part of a reformed EU.