Britain has rarely been in more desperate need of courage and direction from the leader of the opposition. In just under 11 weeks, unless MPs have coalesced around another option, the country will crash out of the European Union, with catastrophic consequences for the union, the economy and its global influence.

There is, however, a window of opportunity, created by a cabinet and Tory party more riven than ever by divisions over Europe, for Labour to shape the UK’s future from the opposition benches. Seldom do opposition parties have as much power to prevent damage to the lives of millions of their voters. Even more seldom do they squander it in the way Jeremy Corbyn has so far.

Five weeks have passed since Theresa May postponed the parliamentary vote on her withdrawal deal. That’s five valuable weeks during which MPs could have shaped what happens next. In fact, the only thing that has changed in that time is that May’s chances of getting her deal through have increased. Her strategy is clear: to leave no deal, despite all its ruinous effects, on the table and let the clock run down until the only choice MPs face is a binary one between her deal and no deal.

The truth is that Labour’s current approach is just as irresponsible as the government’s

The person with the greatest power to thwart May is Corbyn. In recent weeks, we have seen backbench leadership from both sides of the Commons, which, together with a Speaker who has championed the rights of MPs over the executive, has resulted in some important procedural wins in terms of how the next few weeks will unfold. But there remains no parliamentary majority in favour of any one option. While these procedural wins are not insignificant, they are not sufficient to push the government on to a different course. It is difficult to envisage how a group of cross-party MPs, led by backbenchers, could hold together a Commons majority outside the party system for the weeks, if not months, that would be required.

This is why opposition leadership, in the face of a weak and divided cabinet, has never been so critical: the only way to force the government to change direction is through party leadership and whipping MPs behind an alternative position. Yet the truth is that Labour’s current approach – that it wants a mandate via a general election to negotiate a better withdrawal deal – is just as irresponsible as the government’s. There is no realistic route to a general election; May’s DUP coalition partners and her Eurosceptic MPs may not support her deal, but neither will they vote to trigger a general election. The Labour leadership’s pretence that they can negotiate a Brexit that maintains the “exact same” benefits of EU membership while curtailing freedom of movement is sheer fantasy.

Labour must move to support a referendum as soon as she loses the vote on her deal this week, if necessary via the no confidence vote it will surely lose. As we have long argued, a referendum on the deal versus the status quo is right in principle; the 2016 referendum provided a narrow mandate for the government to negotiate the best deal it could, not a blank cheque for it to take Britain out of the EU any way it saw fit, regardless of the costs. The withdrawal agreement that May has negotiated perfectly highlights the Brexit conundrum: there is no deal that lives up to the illusory unicorn the Leave campaign promised voters. There are painful trade-offs and it is up to voters to decide if they want to make them.

Principle aside, a referendum is the only practical alternative Labour can swing behind. The gridlock in parliament only strengthens a pragmatic case for putting the deal to voters. A “Norway plus” Brexit, the other option being talked up, may be the best of all the Brexit options, but it involves Britain sacrificing any say over the rules of the club in which we will effectively remain a member. It will not settle the European question; if anything, having to live by rules set by Europe that we have no influence over will give succour to those Eurosceptics who will campaign to take us further out of Europe and will heighten public hostility to the EU. Moreover, it is difficult to see how Labour can ensure this outcome before Brexit happens on a practical level. Even if the political declaration were redrafted to indicate a Norway-style final destination, it would not be binding on any British prime minister.

The longer Corbyn puts off backing a referendum, the more he creates the impression that he too is simply letting the clock run down in order to avoid making a decision, in the hope that voters will blame Conservatives for any Brexit fiasco. But if Labour enables May’s Brexit, history will not forgive the party. Voters will rightly hold the whole political establishment accountable for a withdrawal deal that will inevitably make Britain into a rule-taker, jeopardise the union and make people poorer.

Both the prime minister and the leader of the opposition are engaged in a dangerous game of brinkmanship

Some have made the case that we should avoid another referendum as it will be divisive, but we should be wary of these arguments. There are indeed risks associated with another referendum, but every path facing Britain at this juncture involves considerable risk of popular backlash. Our political discourse has already deteriorated and a tiny minority with hateful views has become emboldened to the extent that MPs going about their daily jobs now get intimidated, harassed and called “Nazi” by thugs in the street.

But what sort of response is it for a cabinet minister to warn MPs they must back the prime minister or risk unleashing a wave of neo-Nazi extremism, as Chris Grayling did? Since when has it been acceptable for the government to imply extremist violence is an effective way to bring about change, by publicly telling MPs they should vote out of fear of neo-Nazis rather than based on what is right for their constituents? Grayling’s insinuation that Britain cannot bear another referendum without descending into civil conflict is irresponsible and patronising.

Both the prime minister and the leader of the opposition are engaged in a dangerous game of brinkmanship. The longer it goes on, the more certain its conclusion becomes: a Brexit that could split the union and sharpen the inequalities and resentments between the richest and poorest parts of the country. Corbyn faces a choice: he can help prevent it or he can go down in history as having been entirely complicit in making this tragedy happen.