ARTICLE 50: Once the UK leaves the EU, is there any way back in?
And, while politically it is difficult to see such a move happening any time soon, technically there is nothing to stop it.
Fiona de Londras, professor of Global Legal Studies at the University of Birmingham, said: "The UK would have to satisfy the criteria for joining the EU, although given its previous membership this is unlikely to be particularly problematic.
"In sum, these require a state to have a free market economy, a strong democracy, the rule of law, and a willingness to accept all EU laws for negotiations for accession to start."
What would be the legal process involved for getting back into the EU?
Just like any other country seeking membership, Britain would have to apply to join the EU.
"A subject-by-subject negotiation on bringing national laws and systems into line with those of the EU would begin," Ms de Londras said.
"Usually this accession process is a very long one during which the state receives the financial, logistical and technical support it needs to adopt the enormous volume of applicable EU laws."
However, Ms de Londras believes that, having previously been a member of the EU, the process should not take as much time.
"This is why people talk of the UK receiving a 'fast track' back into the European Union, although the euro would probably be a real sticking point," she adds.
"It is very unlikely that the UK would be able to secure the same terms it currently has, including the large number of opt-outs it enjoys in relation to EU law and policy.
"Usually when a state joins the EU it must accept all EU legislation and is expected to adopt the euro.
"The UK could attempt to negotiate an opt-out to the single currency but it is unclear why the Union would treat the UK any differently to other newly joining states in relation to these basic expectations of membership."
What about travel?
Britain would probably be required to join the Schengen (common travel) area and, Ms de Londras adds, possibly be expected to commit to an "ever closer Union" - a goal to which many of the member states are firmly committed.
:: Would the UK be able to secure a better deal if it were to rejoin?
In this scenario, Ms de Londras believes the UK would find itself having to accept a lot of things it currently opts out of.
"Many of these opt-outs were secured for the UK because, as a member of the EU, it had a veto on major treaty changes," de Londras says.
"That is how it secured its opt-out from the euro (when the Maastricht Treaty was being negotiated) and from Schengen."
Things would also be different as an outsider seeking admission.
"As the UK (like all other member states) had the power to disrupt the desired changes of the other member states, it could use that power to secure concessions of this kind to its own policy agenda," she adds.
"The same power would not be available and the UK's negotiating position would be considerably weaker compared to the EU bloc and the individual member states - all of whom could veto the UK's application to rejoin.
"Given the political and other costs of Brexit for the Union, it is reasonable to expect that the EU27 would approach an application for readmission to the EU with caution.
"Taking all that into account, a better deal than the UK's current membership - or even the same deal - seems highly unlikely."