The UK government has tried hard to reassure EU citizens living in the UK - and Britons abroad - that they will be protected from deportation after Brexit.
But many people do not fit neatly into either category, throwing up complex and unexpected dilemmas for both the Home Office and the individuals concerned.
Some, such as Ursula Crosby, who contacted the BBC with her story, have been unable to get a clear answer about how to secure residency rights, and citizenship, when Britain leaves the EU, in March 2019.
Ms Crosby retired to Portugal five years ago after 40 years of working as a physiotherapist in NHS hospitals, in London and Shrewsbury. She says she felt like spreading her wings and "doing something a bit different" while she could.
She is a German citizen but under free movement rules she had the security of knowing she could return to the UK, where her daughter lives and where she still owns property, when she wanted to.
Brexit has changed that. The UK has told EU citizens they will be able to apply for "settled status" if they have been living in the UK for five years or have been granted indefinite leave to remain.
It is unclear how this will apply to Ms Crosby, who was granted indefinite leave to remain in the UK in 1979.
Indefinite leave to remain - despite the name - usually expires if you have left the country for two years or more, as is the case with Ms Crosby.
She has received conflicting advice from UK officials about whether she would qualify for settled status and no-one has been able to tell her what will happen if Britain leaves the EU without a deal.
For all the government's assurances about how easy it will be for EU citizens after Brexit, getting a straight answer on all but the most straightforward of cases is very difficult, if the experience of researching this article is anything to go by.
At one point, Ms Crosby was referred to an independent immigration adviser, who told her she would be all right because Britain would probably stay in the European Economic Area after Brexit - which would be news to Prime Minister Theresa May, who has repeatedly ruled that out.
She is now torn between selling up and moving back to the UK before Britain leaves the EU in March, or risking moving back at a later date, during the 21-month transition period that will follow Brexit, in the hope that she can still secure settled status.
"I am sure I could live in Germany but I wouldn't really want to live in Germany," says the 68-year-old.
"I no longer have any connections there, as such. My parents are dead. I have a cousin in Germany.
"I could stay in Portugal but I am not sure I would want to. It was never my intention to and I still have property in England.
"If I ask anybody who meets me, they all think I am British."
She first came to the UK in 1971, on a work permit, long before EU free movement rules came into force, and was married to a British man for many years.
'Lose the chance'
She now regrets not applying for British citizenship when she had the chance but, like many others, she did not think she needed to.
"There are probably quite a lot of people in this situation," says immigration lawyer Colin Yeo, "people that used to live in the UK, perhaps for a very long time, who were assuming they would be able to come back to the UK because it is a member of the EU.
"They will probably be all right if they come back during the transition period."
But, he adds, many will feel "they have got to come back now or they will lose the chance".
Ms Crosby says she can't understand why, as someone in receipt of a full UK state pension, as well as an NHS pension, she may have to wait five years after returning to the UK to qualify for British citizenship, the same as someone from an EU country who has never lived there.
"I am an honest person, I have no convictions, I paid my taxes and if I go back I will be able to support myself. Am I doing any harm?" she says.
The Home Office said it did not comment on individual cases, but a spokeswoman said: "After we leave the EU, in March 2019, there will be an implementation period that ends on 31 December 2020, during which EU citizens and their family members will be free to live, work and study in the UK as they do now.
"EU citizens and their family members who become resident in the UK during the implementation period will be able to make an application to the EU Settlement Scheme and be eligible for temporary leave to remain and, after five years' lawful residence, settled status, under the terms of the Withdrawal Agreement.
"The rights of those EU citizens arriving during the implementation period will be safeguarded on the same terms as has been agreed for those EU citizens resident in the UK before we leave the EU."
Ms Crosby remains unconvinced by the Home Office's reassurances - and she has written to the department to try to get some clear guidance on what she should do next.
"I am not sure I can rely on the immigration system to relax their very inflexible rules," she says.
"My worst fear is that I sell up here, move back to England and then find out that it would not have been necessary."