The far-right activist Tommy Robinson will march on London under the banner of “Brexit Betrayal” on Sunday 9 December – just two days before the parliamentary vote on Theresa May’s deal. The demonstration represents the real-life, undeniable intersection of the Brexit project and the far right. The response of both the left and the anti-Brexit movement could define them for years to come.

This is a profoundly dangerous moment, and will not be just any far-right march. Robinson, whose real name is Stephen Yaxley-Lennon, has a narrative that makes sense well beyond his usual base – that the Tory elites have betrayed the leave-voting masses. The march has the backing of Ukip, which Robinson is now an official adviser of and is attempting to join. And when the deal is voted down, the far right will be able to claim a hollow but rhetorically useful moment of victory and momentum.

It is quite true to say that Robinson is using the Brexit moment to recruit followers and bolster his credibility in the political mainstream. But that is only a fraction of the story. The narrative of national betrayal and imperial nostalgia is at the heart of Brexit, and always has been. The aim of the Brexit project was always to take the nastiest narratives on immigration, race and nationalism and, with the use of a popular vote, put these ideas on the winning side of history. At the moment, swathes of the left seem content to leave them there.

For two years now, the British left has been trapped in a logic of triangulation on Brexit. The overwhelming majority of the left backed remain in 2016, and the overwhelming majority of Labour members now back a fresh referendum. But as Robinson and Ukip march, many on the left, hamstrung by loyalty to the Labour leadership’s fudge on the subject, will attempt to argue the impossible: that the left should oppose the far right, but accept its greatest achievement.

The Brexit Betrayal march is not some glorious last hurrah of the pro-Brexit far right. It is a sign of things to come. Labour’s position on Brexit has been driven by electoral calculation, but some also raised the argument that continuing to oppose Brexit would boost racism and stoke division. The reality is the precise opposite: the Brexit moment has irretrievably emboldened the far right and its narratives. The genie is out of the bottle. The question is whether the left has both the radical solutions to the social crisis on which it feeds, and the intellectual courage to defeat it.

It is the latter of these that should give us the most cause for alarm. When the likes of Hillary Clinton argue that progressives can beat the far right by erecting more borders, many rightly slam this idea. But in truth, the left is also ceding ground. When a left Labour leadership, and a sizeable chunk of the activist left, will no longer even defend free movement, we cannot be under any illusions that this is anything other than an accommodation to the right.

We are living through a moment of encroaching darkness and nationalist resurgence – and many, both in the centre and on the left are, despite any electoral advances, in intellectual retreat. In this moment, there is no third way. Either we fight the rise of borders, flags, prejudice, nostalgia and rightwing populism, or we will be engulfed by them.

Brexit lacks the moral simplicity of Donald Trump, Jair Bolsonaro and Marine Le Pen. Defeating it requires advocating membership of an organisation – the EU – whose refugee policy and fiscal policy we rightly criticise, and whose role in the world is sometimes far from progressive. But in the cold text of May’s deal and the steady thudding boots of the far right who will march on 9 December, the reality of it can no longer be denied.

The mobilisation of the far right on the eve of the parliamentary vote on May’s deal presents the burgeoning anti-Brexit movement with a test and an opportunity. Just over a month ago, 700,000 people marched for a people’s vote. Another Europe is Possible has, along with a series of prominent left figures, called for an explicitly anti-Brexit mobilisation on 9 December, which will build numbers for a joint general anti-fascist protest. If remainers mobilise, they could dwarf the pro-Brexit far right. For many liberally minded pro-Europeans, this could be a moment of further politicisation, an important landmark on a political journey.

But, above all, what happens now is a litmus test for the British left. Brexit means the biggest expansion of border controls in decades. It will vindicate and mainstream the far right’s narratives on immigration and aggressive nationalism. With a negotiated compromise deal in place, signed by either the Tories or Labour, Robinson’s campaign against an imagined national betrayal will go on indefinitely, nourished by the victory of the wider Brexit project.

We will all stand together on 9 December. But the left now faces a choice. As the far right and the Brexit project intersect, it can connect these dots, and fight against the politics of Brexit while presenting its wider radical vision for society. Or it can stand aside from history.

• Michael Chessum is a freelance writer and national organiser for Another Europe is Possible