OK, I’ll say it out loud. I agree with Nigel. Sorry, I can’t help it. For once, Farage is spot on. The UK’s “winner-takes-all”, first-past-the-post (FPTP) voting system is, to quote the Ukip leader, “totally bankrupt”.

In May’s general election, 3.8 million people cast their votes for Ukip – almost half as many as those who voted Labour and more than double the number who backed the SNP. Yet while Labour won 232 seats and the SNP secured 56, Ukip voters elected a single MP to parliament. So, too, did the Greens, despite amassing more than a million votes nationwide.

So why isn’t electoral reform a higher priority for the left? How can any self-styled progressive support an electoral system that denies millions of voters a voice in parliament – no matter how reactionary or regressive, in the case of Ukip, that voice is deemed to be?

The progressive case for proportional representation (PR), in which the number of parliamentary seats allocated to a political party reflects that party’s share of the popular vote, is pretty straightforward. First, it relates to fairness. Isn’t that what leftwing politics is supposed to be all about? Where’s the fairness in first past the post? In Scotland the 50% of the population who didn’t vote for the SNP elected 5% of Scotland’s MPs while the 50% who did vote for the Scottish nationalists elected 95% of the nation’s Westminster representatives. Is this fair? You must be having a laugh.

The Electoral Reform Society (ERS) has described the 2015 general election as “the most disproportionate in electoral history”. Three out of four votes cast were “wasted” while the majority of MPs were elected with fewer than 50% of the votes cast in their constituencies. The SDLP won the Belfast South constituency with a mere 24.5% of the vote – the lowest winning vote share since records began, according to the ERS.

At a national level, the Tories obtained a Commons majority with 36.9% of the vote – or one in four eligible voters. As Andrew Rawnsley observed: “By no normal definition of the word popular were the Conservatives popular at the election.” In fact, not since 1931 has a prime minister won a majority of the vote to match his or her majority in the Commons. Not Blair, not Thatcher, not Attlee.

The second progressive argument for proportional representation relates to outcomes. If you claim to care about protecting public spending and tackling inequality, then PR is the voting system for you. Don’t believe me? Consider the evidence cited by David Soskice and Torben Iversen in a paper published in the American Political Science review in 2006. “To a very considerable extent,” they wrote, “redistribution is the result of electoral systems and the class coalitions they engender”. The authors discovered that the electoral system has “a strong and statistically significant effect” on levels of redistribution, concluding: “PR systems redistribute more than majoritarian systems” while “centre-left governments dominate under PR systems, whereas centre-right governments dominate under majoritarian systems”.

Just look at the continent: social democracy has thrived across the European Union, where PR and coalition governments are the norm. By contrast, in the UK, FPTP has delivered a succession of majority Conservative governments on a minority of the vote, for much of the past 100 years.

There is an opportunity here for the Labour party, as it languishes in opposition for another five years. But it will require a new party leader with the honesty to admit that, thanks to FPTP, Labour has been forced to chase a few thousand swing voters in a handful of “middle England” marginals, while piling up pointless majorities in safe northern seats. Will they sit on their hands as Ed Miliband, Gordon Brown and Tony Blair did before them, or will they dare to form a belated alliance on the issue of electoral reform with the Greens, Ukip, the SNP and, yes, the next leader of the Liberal Democrats? On Sunday, Tim Farron, the frontrunner for the Lib Dems’ top job, said he planned to use one of his party’s first opposition debate days to lobby for the introduction of PR in elections to the House of Commons, local government and a future, fully elected House of Lords. (The fact that voters decisively rejected the Alternative Vote (AV) system in a referendum in 2011 is irrelevant; AV isn’t PR.)

Yet Andy Burnham, the frontrunner for Labour’s top job, is on record dismissing electoral reform as a “peripheral issue”, as a “kind of fringe pursuit for Guardian-reading classes”. It isn’t.

PR might not have fundamentally changed the result of May’s election – the UK would now have a Conservative-led, rather than a Conservative-only, government – but, in the long run, it could provide Labour with a route back into office. Locked out of Scotland by the SNP and the south of England by the Tories, PR would make Her Majesty’s opposition competitive again at both ends of the UK. To sum up, backing PR allows the Labour party to make a principled case for a partisan advantage. What’s not to like?

• Mehdi Hasan is a presenter for al-Jazeera English. The views reflected here are his own