UK citizens living in some EU countries must apply by Wednesday for post-Brexit residency status, as do EU nationals in the UK.
Britons living in some EU countries are being reminded that Wednesday is the deadline for applying for formal residency status under post-Brexit rules.
British in Europe, a coalition group campaigning for Britons living in the EU, has renewed a warning for people living in France, Latvia and Malta, that they risk losing their rights if they do not do so.
Luxembourg was also due to implement a deadline on Wednesday but last week extended this by six months.
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.
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.
Millions of Europeans have freely lived, worked and studied in other countries on both sides of the English Channel for decades, but Brexit means that those rights are often no longer automatically granted.
Britons living in Spain, which has the highest number of UK residents among EU countries, have been advised to ensure they are correctly registered.
Confusion over UK residents in France
There has been confusion over the deadline imposed by the French authorities for Britons living in France to apply for residency.
Some reports had suggested that the date had been pushed back by several months, and some regional authorities have reportedly been taking such an approach.
But British in Europe has issued strong warnings that the official French deadline remains Wednesday, June 30 and that people risk losing rights if they fail to apply.
The campaign group has been taking to social media, appealing to friends and family members of those who may not have applied to contact them urgently to make them aware they need to act quickly.
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.
Deadline day also for EU nationals in UK
Separately, EU citizens living in the UK have been told they must apply by Wednesday for "settled status", or risk eventually losing their right to live, work, and access public services.
Last week the British government warned that they faced being handed a formal 28-day notice if they failed to apply by June 30.
The United Kingdom left the European Union on January 31, 2020, but most post-Brexit rules only took effect on January 1, 2021, upon the expiry of the transition period.
This includes the end of free movement for EU nationals in the UK and Britons on the continent, although people living in other countries by the end of 2020 have the right to remain.
Wednesday's deadline for residency applications in the UK and the three EU countries marks the end of a six-month grace period.