For those of us on the left, London was witness to some bizarre scenes last Saturday. As a hundred thousand people, maybe more, marched against Brexit, the most memorable chant that could be heard echoing across Parliament Square was not about migrants’ rights or the dispossession of Britain’s young people. Instead, it was “where’s Jeremy Corbyn?” – an inversion of last summer’s Glastonbury hymn.

Many Labour activists have been quick to roll their eyes at what they perceive as a middle-class centrist parade. Corbyn, as it happens, was at a Palestinian refugee camp. But many people on the demonstration didn’t join in with the chant. They were, like me and the vast majority of Labour members, both pro-Corbyn and anti-Brexit – and I want that voice to be heard. Last week I set up a petition for Momentum members, calling on the organisation to oppose a Tory Brexit. It is the first stage of a process that could see Momentum ballot its members on the issue.

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We do not have much time to change the terms of the Brexit debate – both within the Labour party and in the wider country. There is now only one kind of Brexit on offer – the one negotiated by Theresa May. It is set to be put to parliament in October, and it will be an attack on workers’ rights, human rights and environmental standards. Its immediate effect will be not only to sink the British economy, but also to deregulate it and bring us into line with more aggressive neoliberal financial models. It will mean fewer jobs, lower wages and declining living standards for workers – and the effects on the public finances could scupper Labour’s transformative programme.

But for the left, Brexit is about a broader set of processes and ideas. The key narrative on which it is built is about blaming migrants – like me – for a social and economic crisis created by the elite. Brexit is a rightwing project; its key components are nationalism and a drive to get rid of “red tape”. All over the continent, the far right is on the march, attacking refugees, stripping away reproductive rights and challenging the bedrock of peace in Europe. If its narratives become mainstream in the UK, they will steamroller the left just as they have done elsewhere.

The task of bringing back round both the Labour leadership and the majority of members is being made infinitely harder by the behaviour of some in Labour, who not only oppose Brexit, but view it as a useful tool in fighting for control of the party. The entire project of the Corbyn leadership has been built under siege from the centrist establishment – and it is understandable that members are sensitive about that. New Labour and its political acolytes were, as much as anyone, responsible for the deregulation of the British economy, the anti-immigrant politics and the alienation that drove so much of the leave vote in the first place.

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For everyone on the Labour left – from relatively recent members, people who have come back to the Labour fold, or the stoic few who slogged through the years when managerial policy replaced real politics within the party – a radical Labour is the only type we want. We want to see its agenda of massive redistribution of wealth and power, we want it to transform society, and we also believe this is the only platform that can meaningfully bring back working-class leave voters.

I am a migrant worker in Britain, and I see and live through all this. I refuse to be put in a separate category from the British working class, and I refuse to have my position painted as elitist, centrist, or an attempt to destabilise Corbyn. In wanting to stop a Tory Brexit, I am not in a minority either – not on the left, and not in Labour.

The Labour movement is at its best when it is democratic. I hope that pushing Momentum to hold a vote of its members to determine its position will be the beginning of the real Brexit debate within Labour, separate from the political gamesmanship of many MPs. At the same time, dozens of local parties are set to consider taking an explicitly leftwing anti-Brexit position to party conference in September. It is happening late in the day, but the fight for Labour’s Brexit position is now under way in earnest. In it, Momentum and the left must play the leading role.

• Alena Ivanova​ is a​ Momentum and Labour party activist based in London