Britain voted to leave the European Union in June 2016. But support seems to be growing for a second referendum (“people’s vote”) on the terms of Britain’s exit. Fueled by accusations that the first referendum was corrupted by lying, illegal campaign activities, and even Russian interference, leaders and voters from across the political spectrum have lent their weight to the plan for a second poll. Especially in the opposition Labour Party, grassroots support for another referendum appears to be burgeoning.

There are two problems with plans for a second referendum, however: (A) remaining inside the EU is no longer something that Britain can decide for itself, but (B) a second referendum on Brexit would only make sense if an option to remain inside the EU could be included on the ballot. Combined, these structural barriers to a second referendum will probably be enough to make sure that one never happens.

In March 2017, nine months after shock referendum result in favor of Brexit, Prime Minister Theresa May notified Donald Tusk, the president of the European Commission, that Britain intended to withdraw from the EU. This notification was given as per Article 50 of the Lisbon Treaty, which sets out the procedure for member-states being able to leave the EU. With that act, Brexit was set in motion and transformed from a domestic question into an active international process.

The Lisbon Treaty affords Britain and the EU two years to negotiate the terms of Britain’s exit, only extendable by unanimous agreement. If both sides fail to conclude a mutually acceptable withdrawal agreement in this allotted time, then Article 50 provides that membership of the EU will simply cease to apply to Britain. Crucially, there is no explicit provision for halting or reversing the withdrawal process: exiting states are locked into leaving the EU one way or the other.

This means that Brexit has been an international process since March 2017. It is governed not just by domestic legislation, but also by international law. Decisions involve not only the British people and Parliament but also the twenty-seven other EU governments and the EU's supranational institutions. In short, there is almost nothing about Brexit that can now be decided by Britain acting unilaterally.

Given these constraints, is it possible for a second referendum on Brexit to be held? The answer depends upon what options are to be put before the British people.

One idea is to ask voters to accept or reject whatever withdrawal deal is concluded between London and Brussels. This, however, would be an almost complete waste of time. If the British people refuse whatever agreement is reached between Britain and the EU, then the default outcome would be Britain crashing out of the EU with no arrangements in place to soften the landing. Nobody wants this outcome, and so it makes little sense to have a referendum where only one option is desirable or practicable. As it stands, Parliament will have to ratify the withdrawal agreement, but this will be a formality. Rejecting the deal—any deal—will not be a practical option.

The more ambitious alternative is to have a referendum where remaining inside the EU is included on the ballot. Presumably, such a referendum would ask voters to choose between accepting the finalized exit deal or voting to stay inside the EU. For this to happen, however, both options being put before the electorate would have to be viable. That is, the public must be offered two options that the British government has the power to implement. Yet arriving at a point where the British government can credibly put these two options before the electorate—accept the exit deal or choose to remain inside the EU—is almost certainly never going to happen.

Consider what would have to be done. First, the EU would have to agree (unanimously) to extend the two-year negotiating period for the express purpose of negotiating two separate outcomes for the British people to choose between. Britain and the EU would then have to continue their current negotiations on an exit deal (which have been ongoing for eighteen months and have produced almost nothing of substance) while simultaneously negotiating the terms on which Britain could remain inside the EU. Only once both alternative pathways have been agreed upon could a referendum be held. This process could take years to achieve given the difficulty of finding agreement among the twenty-seven other EU governments and given that Britain's civil service is already severely overstretched.

Some argue that it would be easy for Britain to remain inside the EU. They claim that voters could choose to return to the status quo ante, and so there is no negotiating to be done on this front. But this is wishful thinking. It is not up to Britain to decide whether it can remain inside the EU on its previous terms. This is an open question, to be resolved either by a legal ruling from the Court of Justice of the European Union or through a political agreement in the European Council. Either way, it is not a decision for Britain to make alone.

If Britain’s right to remain inside the EU is to be decided as a political question, the key sticking point would be whether, because the Article 50 withdrawal process is already underway, Britain would have to reapply for membership of the EU just like any other would-be member-state. Some EU leaders such as Jean-Claude Juncker, the president of the European Commission, have suggested as much. But reapplying for admission would pose some knotty questions for negotiators, and almost certainly would not be a quick and smooth process.

The problem is that, for decades, Britain has enjoyed special treatment inside the EU—specifically, a rebate of financial contributions and opt-outs of the European single currency and the “no borders” Schengen travel agreement. It would be essential for Britain to keep these privileges as part of any deal to remain part of the EU. Nobody believes that the British people would agree to stay inside the EU on worse terms than before, and so there would be no point whatsoever in holding a referendum where such an option was included on the ballot.

Lengthy negotiations await, then. But as if the substance of these two parallel sets of negotiations was not thorny enough, the whole process would depend upon the EU maintaining unanimous agreement in favor of extending the negotiating period. This will be difficult to achieve given that leaders of the rest of the EU members each have their own domestic politics to consider. And, of course, the current British government must remain in place. Given that Theresa May currently heads a minority government, this cannot be taken for granted. What would happen if the government falls? What happens if there is a General Election? Would the EU keep agreeing to extend the negotiating period while Britain forms new governments and new negotiating positions? For how long could such a situation be allowed to persist? At what point does Britain have to make up its mind?

There are just six months remaining before Britain exits the EU, with or without a withdrawal agreement in place. Is it strictly impossible that the movement for a second referendum can put in place the political conditions necessary for a meaningful second vote? No. Anything is possible in international politics. But it nevertheless borders on the fanciful to suggest that Britain’s leaders will change their minds on Brexit, persuade all twenty-seven other EU governments to extend the negotiating period indefinitely, ask the EU for an option to remain, negotiate the terms of two possible future relationships, legislate for and organize a second referendum on Brexit, and then implement whatever pathway the British people choose.

Even with the best will in the world, the fact remains that Britain no longer has unilateral control over its membership of the EU. It gave up that control when it triggered Article 50 in March 2017. The British government can still shape the terms of Brexit, of course, but it cannot decide those terms for itself. Also, it cannot choose to halt Brexit altogether. These are questions upon which the rest of the EU gets an equal, if not more important, say. As of now, finding consensus on anything other than a barely acceptable withdrawal agreement looks desperately unlikely.

Peter Harris is an assistant professor of political science at Colorado State University. You can follow him on Twitter: @ipeterharris.

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