Whatever the outcome of the EU referendum, there will be voters who, after the event, will have second thoughts on the wisdom of their choice. They will have cast their vote, however. And that cannot be undone. The result will stand.
Assuming the vote is to leave, withdrawal negotiations would ensue and the UK would probably leave the EU two years later.
But what if after leaving, the UK had a change of heart? What if the economic doom predicted by some in the Remain campaign were to be realised? Could the UK simply rejoin the EU?
This would be uncharted territory because the UK would have been the first member state to leave the EU. Nevertheless, Article 50 of the Treaty on European Union is clear on the matter:
In other words, if the UK wanted to rejoin, it would be treated no differently to any other state seeking membership. There is no special procedure for readmission. There is no special route back into the EU for past members.
No picking and choosing
So if the UK wanted to rejoin the EU it would have to submit an application for membership as if it were applying for the first time.
The EU would review whether the UK meets the membership criteria. Given its previous membership of the EU, it is extremely unlikely that the UKs eligibility would be questioned.
But the UK would also need to show that it is ready to assume the obligations of membership. And that is a different matter.
A key factor here is the nature of the membership that would be on offer. Importantly, precedent from other states joining suggests that the UK would have to accept full membership. It would not be able to insist on the same opt-outs and the budget rebate it currently enjoys.
That probably means signing up to the Schengen area on passport-free movement and, importantly, committing to joining the euro. It would also mean recommitting to the goal of ever closer union.
Importantly, current opt-outs were not secured through accession to the EU. The UK opt-out from the euro dates back to the 1991 Maastricht Treaty and the opt-out from Schengen came later that decade. In both cases, the UK held a veto over treaty reforms under discussion at the time. Threatening to use that veto, it extracted these concessions.
Beware the veto
If the UK, having left, were to seek to rejoin the EU, it would not be in a position to wield a veto. The power of veto would lie very much with the EU and its member states.
In fact, the unanimous agreement of the member states is needed to admit a new member. Each has a veto, as the UK first found to its cost in 1963 when French president Charles de Gaulle issued an emphatic non to the UK joining the European Communities.
As the case of Turkey amply demonstrates, each member state holds multiple vetoes to a country joining the EU. They can veto the opening of negotiations, the opening and closing of each of the 36-plus negotiating chapters, and the signing of the treaty of accession.
It would take much less time for the UK to negotiate accession than it is taking Turkey, but the process would not be swift and would not automatically lead to membership.
It would also most likely be frustrating. All the evidence shows the EU is increasingly strict in its application of the accession criteria and holds rigidly to the position that there can be no opt-outs, exemptions or permanent derogations from the obligations of membership.
There is also a clear expectation that an applicant state demonstrates a proven track record in implementing the acquis before it is admitted.
In the specific case of a further UK application, it could be anticipated that the EUs member states and institutions, having facilitated a UK withdrawal, would also need serious reassurances (and probably sustained evidence) that the UK was committed to membership before they would be willing to readmit it. Having lost an awkward partner, they would be unlikely to hold the door wide open and unhesitatingly welcome the UK back with open arms.
In the interests of securing renewed UK membership, the EU might be willing to readmit the UK on the same terms as it had prior to withdrawal, with exemptions from Schengen and the euro, but that is extremely unlikely. There would be no requirement to honour the previous terms of membership. After all, a UK vote to leave the EU would have been a democratic rejection of those terms.
More importantly, granting an opt-out would reignite the possibility of contagion. Other member states might seek special treatment, too. It is also almost inconceivable that newer member states who have assumed all the obligations of membership would be willing to see a larger and richer state being granted special treatment.
Even if all these hurdles were cleared, most member states also need to get the agreement of their national parliaments before a new country can join. Some, such as France, could even have to hold a referendum.
Joining the EU is a complicated, demanding and highly politicised process. Rejoining the EU would be no different. It would take time and require unanimous support from existing member states.
Moreover, there is no guarantee at all that the previous terms of membership would be either acknowledged or honoured. A leave would be a leave. Rejoining the EU would involve applying afresh and being subject to the same principles and procedures as any other state seeking to join.