EU nationals living in the UK and Britons in some countries on the continent face an end-of-June deadline to apply to formalise their status, but many have yet to do so.
Campaigners for EU nationals living in the UK and Britons living on the continent have issued calls for people to act now to secure their post-Brexit residency status before application deadlines expire at the end of June.
Under the terms of the Brexit divorce deal, EU citizens already living in the UK and Britons living on the continent by the end of 2020 can remain with guaranteed rights, with the right to apply for permanent residence after five years.
Failure to do so would mean that from July 1 people would lose essential rights they had enjoyed up to now — such as the right to work, rent accommodation, or receive some hospital treatment.
British in Europe, a coalition group campaigning for British nationals living in the EU, also took to Twitter to remind those living in four countries — France, Luxembourg, Latvia and Malta — that they have until June 30 to apply, or "face waking up as undocumented immigrants" the next day.
The Netherlands has extended its deadline to October 1, while over a dozen EU countries are implementing systems automatically granting legally resident Britons residence status.
By March 31 this year, there had been 5.2 million applications to the UK's settlement scheme for EU citizens, according to the think tank The UK in a Changing Europe. It said the numbers showed EU movement to the UK to be "much higher" than previously thought, adding that "we simply do not know how many EU nationals need to make an application... before the deadline".
The3million group points out that from July, EU nationals in the UK will only have digital proof of "pre-settled" or "settled" status. It has argued that the lack of paper document leaves many people vulnerable, and there have been many complaints about how the scheme has worked in practice.
The UK government has said that late applications will be considered where there are "good reasons" for missing the deadline, issuing guidance in April as to what such reasons might be.
Nevertheless, "it has been confirmed that those who miss the deadline will become, overnight, unauthorised/illegal migrants, subject to the hostile environment and possible removal," concluded Professor Charlotte O'Brien of the support group EU Rights and Brexit Hub.
Regarding Britons living on the continent, a joint EU-UK report on the implementation of residence rights published in April found that fewer than half of all UK nationals living in France and Malta had applied for and received a decision on their applications.
Nearly one in five people living in countries with a June 30 deadline had not even applied, figures which British in Europe described as "shocking and should be a wake-up call" to member states.
It criticises EU countries for failing to deliver on pledges contained in the Brexit divorce deal to provide adequate information and raise awareness among British residents and calls on the EU and the UK to recognise their responsibilities towards them.
"However successful the implementation is, there will be a proportion of people who fall through the gaps and become undocumented in the process, with serious implications for their lives that may start to come to light later", said Professor Michaela Benson of Lancaster University, a specialist in post-Brexit citizens' rights, after giving evidence to a House of Lords committee last week.