When Greenland left the European Community in the 1980s, the legacy rights of expatriate citizens were guaranteed with a legal act of extraordinary simplicity: the operative article runs to just 85 words.

The fate of 4m EU and British expatriates looks far more uncertain and complex. One senior Brexit negotiator fears talks on citizen rights could sink into “a horrible legal morass”.

“It’s immense. Every time you think you’re across it, you turn another corner and find another mess,” said another senior EU diplomat.

Both London and the EU-27 agree on a broad goal: a reciprocal deal to guarantee the rights of 3m EU citizens in Britain, and about 1m British expats within the union. But within the detail of that superficial agreement lies an expat lifetime’s worth of politically sensitive choices.

Residence definitions, pension rights, unborn children, the ability to move, claim benefits, marry, divorce, even commit crime and avoid deportation — an entire cycle of modern life is potentially touched by the Brexit agreement.

“We can’t hide the fact that it is complex,” said Guy Verhofstadt, the European Parliament’s chief Brexit negotiator.

“But if we base ourselves on the principles of reciprocity and a uniform approach by the EU-27 then in my view there is no reason why we cannot find a lasting and equitable solution.”

Here the Financial Times runs through seven obstacles to a deal.

Nothing can be taken for granted

There is no systematic register of when expats arrived in their current place of residence. Worse still, EU officials think it is not feasible to create a comprehensive one before the expected Brexit date in 2019. So even if some rights are guaranteed, citizens will need to prove eligibility. Britain’s 85-page residency form offers an insight on the bureaucracy ahead.

Not all expats are the same

No British politician has questioned the right to remain of legitimate EU migrants. But that is only one small sliver of EU citizen rights. At issue are work rights, welfare access, health provision, discounted student fees, even the ability to draw a UK state pension 50 years from now. And these rights depend on circumstances.

Under EU law, migrant workers have different rights to students, pensioners or jobseekers. Residing in a country for more than five years — and thereby gaining “permanent residence” — is an important threshold for gaining rights. But decisions will depend on proof.

Circumstances change

Managing change will be hard. If an expat marries, what rights would their partner enjoy, and would their nationality matter? Could they bring in-laws to the country? Similarly an expat’s legal status may evolve over time, should they lose their job or move country.

The law, too, will evolve. The EU may legislate to change rights post-Brexit. EU nationals may want to challenge Britain’s application of their rights. Would that be in British courts or European courts? And whose interpretation of EU law would prevail? A big role for European courts would cross a red line for London.

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The two sides want to guarantee different rights

The EU-27 want to maintain full rights for EU expats but this could be tricky for Theresa May, the prime minister. If existing welfare rights remain intact, for instance, EU migrants could still claim UK child benefit for dependants in Paris or Warsaw — long a British tabloid bugbear. Full EU rights would also restrict Britain’s ability to deport an EU migrant who has committed crimes after Brexit.

A third example is pensions. At present a Brit moving to Australia can draw their UK state pension, but it would be frozen, and not increased in line with inflation and earnings. A Slovak or German migrant worker who leaves Britain, by contrast, would enjoy better rights: their full UK pension drawn overseas and uprated every year.

There may need to be dozens of deals

Brexit poses two questions on citizen rights: the legacy rights of current expats and what terms future expats may enjoy.

Britain may seek to tackle both in one deal — reciprocated by the EU. That would apply a single, probably less generous, regime of rights to all present and future EU migrants, covering health, benefits and citizenship.

Potential problems will arise from watering down EU rights. That is because the EU-27’s first and foremost concern is preserving full rights for the existing 3m migrants, rather than future flows.

Depart too much from the EU’s legal baseline and EU negotiators warn a citizen rights deal may not be possible under the Article 50 exit clause. Instead country-by-country bilateral deals may be necessary with each of the 27 members. That is hard to negotiate and ratify, and even harder for expats to understand and apply.

Beware the cut-off point

No Brexit deal on rights can be open-ended. Negotiators are looking at various cut-off points: the lifetime of the eligible expats; a period of time, say five or 10 years; or until the point at which the expat gives up their enhanced rights by moving country. All three options have political and technical upsides and downsides.

One complication is that some rights, such as pensions, will be for life and even cover former expats. Then there is the eligibility date. British ministers want to draw a line on EU free movement rights and are looking at three options: the point of the Brexit referendum, the Article 50 notification, or Britain’s exit. EU-27 negotiators see nothing to discuss: EU rights and obligations continue until the point Britain leaves.

Early, late or hard?

Expat rights will be one of the first topics to be discussed in Brexit talks. Both sides want a quick deal but that may be impossible. Diplomats are scrambling to work out what would happen in the event of no deal. Expats would basically be at the mercy of national governments.

But there are some protections. EU law does cover safeguard rights for some third-country nationals. And a raft of dormant UK bilateral agreements with European countries on welfare — superseded by EU membership — may be revived. One such agreement dug up: a 1923 Anglo-Finnish treaty on “the disposal of the estates of deceased seamen”.