This evening, after two days of heated debate in the House of Commons, MPs will have their first formal say on the triggering of article 50. It’s a seismic moment – and will define this country for years to come.

There will be two votes tonight: the first on a “reasoned amendment” tabled by the Scottish National party with the support of the Green party, Plaid Cymru and the Social Democratic and Labour party. A reasoned amendment – for those unfamiliar with parliamentary language – aims to strike out a bill entirely, and in this case parliament voting in favour of it would stop article 50 being triggered at this stage. If that vote is lost, there will then be a vote on whether the bill proceeds to the next stage.

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I put down my own reasoned amendment to the bill – and was pleased to have support from MPs from several parties – but it’s in the gift of the speaker, John Bercow, to select which one to put to debate and a vote. The Labour party leadership did not put forward any such amendment because they plan to walk through the voting lobbies with the Tories tonight.

Labour’s decision is extremely disappointing, particularly now that the government’s plan for leaving the EU is becoming frighteningly clear. A Conservative post-Brexit Britain would be “free” from Europe as we drift towards being a tax haven floating in the Atlantic, desperately begging for friendship from a divisive and dangerous US president to whom the prime minister is patently unable or unwilling to stand up. We’ll be out of the single market, out of the customs union and free movement will be stopped.

Indeed the Conservatives’ extreme Brexit essentially means sacrificing membership of the single market at the altar of limiting free movement. The economic risks are huge and our public services are at threat from a government willing to engage in a race to the bottom on corporate taxation. The NHS is suffering badly now – imagine what it will look like without EU nationals working in it, and if the government cuts taxes even further and has even less money to spend on it.

Such a vision for Britain’s future is chilling, but it is not inevitable if those who want something better are prepared to put aside their differences in the name of cooperating to get the best deal for the country. Labour’s failure to participate in this process has handed the Tories a massive advantage. They are set to benefit hugely from rushing through the triggering of article 50 with as little dissent as possible and it’s given them far more opportunity to morph a narrow referendum result in favour of leaving the EU into an overwhelming mandate to depart from the world’s biggest trading zone, curtailing our social and environmental protections along the way.

By promising early on that they would vote unconditionally to trigger article 50, the Labour party has capitulated to the government, as well as reduced its bargaining position on the things they want to secure from the bill. Labour’s amendments to the detailed content of the bill represent important safeguards against an extreme Brexit but their strategy of unconditional support delivered via a three-line whip, removes all incentive for the government to negotiate and thereby undermines their best chance of securing the changes we all want.

After tonight, assuming we don’t see a last-minute change of heart from Jeremy Corbyn, the bill will pass its second reading and proceed to the committee stage, when MPs will have a chance to scrutinise and amend it in much more detail. I’ve tabled a number of amendments myself – specifically aiming to retain our environmental protections, requiring the government to set out a detailed plan for transitional arrangements, and give the public a proper say on the outcome of the negotiations through a “ratification referendum”.

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I’ve also signed a number of amendments tabled by other MPs. From Harriet Harman’s amendment which protects, among other thing, EU-wide action on violence against women, to Chuka Umunna’s to secure spending on the NHS, to Chris Leslie’s ensuring Britain’s continued participation in the European Environment Agency. The Labour leadership’s early white flag waving on Brexit limits the chances of a united opposition passing these amendments, but they remain our best hope of softening the hard Brexit planned by the government, and I’ll be voting for every single one which will better protect this country from falling off an economic and environmental cliff edge.

As the co-leader of a party that stands for environmental, social and economic justice, I will not support a government offering no assurances to EU nationals living in Britain, threatening the funding of our public services, and planning to end our membership of the single market and customs union.

It has been heartening to see such cross-party cooperation on these key issues. I very much hope Labour will rethink their three-line whip on the vote tonight, and that individual MPs will put principle before party loyalty, rather than cast a vote for extreme Brexit. Whatever the outcome, however, I’ll remain open to working with MPs from all parties to amend this bill – that’s the only responsible option if we want to protect this country from the very real dangers of triggering article 50 on the terms Theresa May has so far revealed.