Labour has just suffered its worst election result since 1935; as a result we will all suffer at least a further five years of Tory rule.

But it could have been so much worse for Labour had we – and the Liberal Democrats – not provided our helpful and unrewarded service of stepping aside in some seats where Labour might have made gains and mostly focused our campaigning away from Labour’s more vulnerable seats. Many Greens also voted tactically, and in some cases called on Green candidates not to stand, advocating against our party’s agreed strategy in the middle of an election campaign.

It’s time for us as a party to examine whether backing up Labour in this way is really in the interests of either our party or the country. It’s time for a fundamental strategic examination of our relationship with the Labour Party.

I stood in Stroud because it is a seat with thousands of Green voters who deserve representation. I became a target for Momentum precisely because I was challenging the idea that we somehow owe it to Labour to help them, even though they repeatedly repay this favour by doing their best to eradicate the Green Party.

Labour’s rage was irrationally focused on the Stroud party’s decision to stand their best candidate in a seat with strong Green support. To an independent observer this is how political strategy works: you play your best piece in the place where it is most likely to succeed. This was a challenge to local Labour who work with us on the district council but felt we should confine our power to the local sphere. It culminated in the absurd spectacle of a Labour activist shouting at my communications manager: ‘But why have you chosen such an excellent candidate?’

It must be said that Labour’s communications strategy and the bussing in of thousands of Momentum activists defeated us this time. While we trebled our vote, Green voters were successfully manipulated into tactical voting and deserted us in droves. But I hope that what happened in Stroud is a sign of things to come and an indication that in future we will make our electoral decisions in our party’s interest, not to suit Labour.

To succeed electorally we must challenge the habit of many Greens to vote tactically for Labour in general elections. Tactical voting turns a deeply serious process into an agonising horse race. It has become a threat to democracy itself – a chance to bully those who reject the ineptitude of both the main parties by declaring which parties have a right to run. And the first-past-the-post system provides the perfect opportunity to kneecap your opposition before they even get onto the field.

Tactical voting allows Labour to hide its true weakness and unpopularity, as in Scotland before its final collapse. Challenging Labour would encourage it to do better rather than, as Gaby Hinsliff describes, allowing it to behave ‘as if anti-Tory voters simply had a moral obligation to vote for them, trumping their own responsibility for confronting any shortcomings’. Labour cannot expect competitors to step aside: it should itself work harder to win voters back from the Tories.

Tactical voting is also a con. In every seat except Putney, tactical votes for Labour failed to remove Tories so, tragically, tactical votes were also wasted votes. And tactical voting masks voters’ true intentions. How would the outcome of the last couple of elections have looked if Greens had not quietly stood aside or dialled down our campaigns in Labour marginal constituencies, explicitly accepting tactical voting? For this we have gained nothing but mockery from Labour activists at the small size of our vote – and so the diminution of the Green vote is perpetuated. This was the taboo that we dared to challenge in Stroud this time around.

Labour have to stop treating the Greens as an inferior sidekick, a source of good ideas that they can pass off as their own, and an inconvenient rival on the left to be patronised rather than respected. As Greens we need to forcefully reject the idea made explicit in Paul Mason’s recent strategy document that we should be absorbed by Labour. We need to reject offers limited to dialogue and influence and instead demand a fair share of power commensurate with our vote share and our popularity. We must demand this as a political party, rather than a pressure group, and as part of a strong global political movement.

Most importantly, we must work with those in Labour who are genuinely committed to pluralist politics and sharing power with other parties. As part of his leadership bid Clive Lewis has been clear about his support for proportional representation (PR): “A majority of the British public voted for parties of the left or the liberal centre. But this was in no way reflected in the election result. Labour should have committed itself to changing the voting system decades ago, and we have condemned some parts of our country to 40 years of decline by failing to do so.”

The most important thing we can do to progress Green politics over the next five years is to build a strong cross-party campaign for a renewed democracy and a proportional electoral system. This will mean working with Labour MPs who support PR and challenging those who do not.

We can also pressure all local Labour parties to take pro-PR motions to their conference to shift the party’s position. We must take every opportunity to expose Labour as the only socialist party in Western Europe to support a system that does not fairly translate votes into seats.

When establishing ourselves as a powerful electoral force requires us to take on Labour we should not be afraid to do that. Because while many Greens believe Labour is our friend, their tactics in elections show clearly that they see us as their key opponent. We must act with the wisdom to make strategic decisions in the interests of the politics that we represent and no longer allow a failed Labour Party to set the terms of our electoral strategy or ambition.

Molly Scott Cato is Green Party of England and Wales Member of European Parliament (MEP) for South West England and Gibraltar