This Thursday, voters face an important choice. It’s not the choice they deserve: whether to approve or reject the Brexit deal Theresa May has negotiated on their behalf. But it is the most important opportunity yet to send a clear message to our political leaders that the gridlock, the lacklustre leadership and the general sense of malaise that have infected Westminster since 2016 just isn’t good enough.

For months, people have been denied their say, despite the prolonged parliamentary stalemate and the knots in which both main parties have tied themselves over Brexit. These European elections should not be treated as a proxy referendum: there is no distinct question being put to the public and turnout will probably be much lower than in a general election or referendum. But they are a precious chance for those who share the Observer’s view that Brexit would be an unmitigated disaster to show Labour and the Conservatives exactly what they think of their takeaway from the dire local election results earlier this month – that, somehow, voters who defected to the Lib Dems and Greens were just telling politicians to get on with Brexit.

There are those in the Labour party who maintain it is a Remain party. They are deluding themselves

These EU elections should be about one question and one question alone: what happens next with Brexit? Voters face no dilemma about whether they should vote tactically for Labour to keep the Tories out of Westminster or their local council, or about whether a vote for one of the smaller parties is a vote wasted. The voting system is proportional, and these elections are anything but a straight-up fight between the two main parties. We urge voters to cast their ballot for one of the five parties explicitly in favour of a confirmatory referendum: the Lib Dems, the Greens, Change UK or, in Scotland and Wales, the SNP or Plaid Cymru.

There are those in the Labour party who maintain it is a Remain party. As much as they may wish this to be true, they are deluding themselves. Labour is a Brexit party, under a Eurosceptic leader who has unequivocally committed the party to trying to deliver Brexit.

The Observer has been consistently critical of May for absorbing the deceit of the Leave campaign; for never levelling with her MPs and the public that Brexit was going to involve painful trade-offs. Her strategy has been shaped only by her desperation to keep some semblance of unity in the Tory party, not by the national interest.

But from the opposition benches, Labour has been little better. Jeremy Corbyn has fallaciously maintained that if only he were leading the negotiations, he could somehow get a deal that secured all the benefits of membership without any of the costs. Both main parties treated a victory for Leave as a victory for Farage-style populism: just keep up the illusion that Brexit done right could be smooth and pain-free. And so both have aided and abetted the rise of the Brexit party: is it any wonder that some voters are asking: if Brexit really is so simple and downside-free, why hasn’t it yet happened?

Corbyn was elected Labour leader partly on a platform of rejuvenating party democracy. But only, it would seem, when it suits him. According to the party’s deputy leader, an overwhelming majority of MPs back a referendum on any Brexit deal and more than seven in 10 party members want to see Labour back a referendum. But Corbyn is ignoring all that, hoping that merely incorporating a potential referendum into party policy as a third preference, after a Labour Brexit deal that doesn’t exist and a general election Labour doesn’t have the votes to deliver, will be enough of a sop to Labour’s Remain voters to prevent them deserting in droves.

Both main parties have aided and abetted the rise of the Brexit party

It’s not just insulting – it’s foolish. Labour should swing behind a referendum first because, as the Observer has long argued, it is the right thing to do: voters were not offered a firm proposition for how we should leave the EU in 2016; instead, the Leave campaigns sketched out a fantasy Brexit that would involve Britain seizing back control, freeing up vast pots of cash for hospitals and schools, all while delivering a big boost to the economy. Now a concrete deal exists – that commits to a treaty the trade-offs Brexit must involve – it must be put to voters.

But it is also the sensible thing for Labour to do electorally. Its ambiguity over Brexit is no longer chiming with voters. Polls suggest that the majority of Labour’s 2017 voters are planning to defect to other parties on Thursday and that three times as many are Remainers deserting Labour for Remain parties than Leavers for Leave parties. They are right to defect. Corbyn seems determined to interpret a vote for Labour as a positive vote for his Brexit policy. Voting Labour will simply engender more complacency that anti-Brexit voters can be taken for granted despite making up the majority of the party’s supporters. A referendum is unlikely to happen unless Labour’s leadership actively back one. But for months, Corbyn has ignored the principled and pragmatic arguments for Labour to do so. The only viable way to get him to reconsider is to deny him your vote on Thursday.

There are several options. The Observer could not endorse every Green policy but enthusiastically backs their message about the urgency of the climate crisis. In coalition with David Cameron, the Lib Dems enabled five years of punitive and inequitable public spending cuts, but polling suggests they are the strongest national anti-Brexit party, with the best chance of catching up to Labour. While many are justifiably disappointed in Change UK’s overly tribal political tactics, their MPs undoubtedly did a brave thing in rejecting their former parties over their Brexit handling. And though the Observer is no supporter of Scottish or Welsh independence, the SNP and Plaid Cymru have consistently been strong anti-Brexit advocates for Scotland and Wales.

Brexit is the most important question facing the UK for decades and at last voters have an opportunity, albeit imperfect, to deliver a verdict. Labour has backed its anti-Brexit voters into a corner. We urge them not to vote Labour on Thursday, but to use their ballot to make clear to Corbyn that if he does anything to facilitate Brexit in the coming months, it won’t be in their name.