Britons might be able to keep their EU citizenship and rights to live, work and claim healthcare across Europe, even if Theresa May walks out of the negotiations with no deal.
This could depend on the outcome of a legal case brought by four plaintiffs, who are seeking a ruling from the European Court of Justice over whether Article 50 can be revoked without the permission of other EU states.
The case, brought by the Good Law Project, is an attempt to get a ruling over whether Brexit could be reversible until 29 March 2019.
But it will also be asking whether or not UK citizens would remain EU citizens post Brexit.
The argument is based on Article 20 of the Treaty of Lisbon, which states that EU citizenship is additional and separate to national citizenship.
There are no provisions for removing this citizenship and its associated rights from individuals, regardless of whether their nation leaves the EU.
The case will argue that it is unclear from current legislation whether UK citizens can be stripped of their EU citizenship.
Speaking to Buzzfeed, Jolyon Maugham QC, a lawyer helping to bring the case, said: “There seems to be an assumption – convenient both to a particular type of Brexiter and to those voices in the EU that would rather be shot of the UK – that the citizenship rights of UK nationals can be taken away from us.
"Whether that assumption is right is ultimately a question of EU law. And it's very unclear to us that it is.
"The question is likely to be of particular importance to those – very often British pensioners – who have made their lives abroad in France or Spain."
More than £70,000 was crowdfunded for the case and the Good Law Project is hoping a hearing can take place in June.
If the court ruled that EU citizenship could be revoked only with a prior amendment to the treaties, then this would help the UK’s negotiating position a great deal.
This could mean, regardless of the nature of the deal or whether there is a deal at all, that UK citizens could benefit from the same rights as those living in the EU, such as free movement, access to study and work anywhere within the union.
This comes after a leaked draft legislation due to be put to the European Parliament is expected to call on MEPs to preserve EU citizenship rights to UK citizens once Britain leaves the union.
But some legal experts told Buzzfeed this would be an optimistic interpretation of the EU treaties and that more likely, UK citizens would be automatically stripped of EU rights when leaving the union.