Government plans to end freedom of movement immediately if there’s a no-deal Brexit have alarmed many inside and outside the UK.
UK nationals living in EU countries will most likely have to get residency permits, arrange for healthcare and driving permits for the countries they live in.
A blog post by the Home Office published on Thursday said that the Foreign Office was working with countries across Europe to ensure UK nationals residing there would be informed about any changes they would need to make ahead of Brexit.
However, the co-chair of the citizens' campaign group British in Europe, Jane Golding, told Euronews that the reciprocity principle has people very concerned about their fates living in the EU.
"It's about how that announcement will impact the EU 27's approach to all of us British people living in the EU27," she said.
"You have countries that have not yet decided what the final status is going to be and obviously when we talk about reciprocity what the EU27 will be looking at is how their citizens are treated in the UK.
"It could be that countries in the EU27 decide to wait and see what the UK does and that just leads to more and more uncertainty for citizens."
For Golding, "the UK often forgets that free movement goes both ways, so it's not just about people going to the UK, it's also about British citizens using free movement to go and work in other countries".
The European Commission claims it is “putting citizens’ rights first” in its contingency plan for a no-deal scenario. It calls on EU countries to be "generous and pragmatic" in granting temporary residence to UK citizens already living in the bloc on exit day.
But at the European level, there are no guarantees. On social security, Brussels urges member states to “take all possible steps to ensure legal certainty” and protect previously existing rights.
Golding said people were concerned about lacking proof of residency in their respective EU countries during the "grace period" given to regulate their legal statuses.
"You will have a grace period [after Brexit] during which you can apply for your long-term status but until you get that if you haven't got a document to travel, then how do they at the border differentiate between those who were already residents and those who coming post-exit — that's a key issue."
Golding took France for an example where there is no compulsory registration system, leaving people with no documents to prove they were living in there before Brexit.