E.U. supporters called for the United Kingdom to remain in the 28-nation bloc during a rally in London on March 25. (Facundo Arrizabalaga/European Pressphoto Agency)

Twenty-one years ago, Patrizia Mayall moved to Britain after falling in love with a young Englishman serving in the Royal Air Force. She studied linguistics before taking time out of the ­workplace to raise their British children. She places huge value on politeness and tolerance and insists she enjoys queuing and the stand-right, walk-left rule on escalators.

For all appearances, she is British.

Only she’s not. She is an Italian national living in Britain, her ­future thrown into doubt following Britain’s decision to leave the European Union.

“I’ve lived here most of my adult life. I feel more British than Italian,” said Mayall, 45, a bubbly brunette who says she hasn’t had a proper night’s sleep since last summer’s Brexit vote.

Mayall is able to live in the United Kingdom because it is a member of the European Union, a bloc of 28 countries that share freedom-of-movement rules. For decades, this has been the way of life in Europe: People can up and move to another country at a moment’s notice. Some decide to plant roots in their new homeland, acquiring families, jobs, pets and mortgages along the way. Aside from voting, the E.U. citizens who live here enjoy most of the same benefits as Britons.

But with Brexit looming, the future for millions now hangs in the balance. Figuring out what happens to an estimated3.2 million E.U. citizens living in the U.K. — and the 1.2 million Brits living in Europe — will be the most high-profile aspect of early Brexit talks as the two-year exit process gets underway.

British Prime Minister Theresa May insists that she wants a quick solution — alongside a reciprocal arrangement for Brits on the continent — but no one knows what that will look like.

Analysts say the vast majority of E.U. citizens living here will have their right to residency confirmed, but it’s likely there will be some who fall on the wrong side of the line. How will those who live here go about proving their right to live and work in the U.K., thus differentiating themselves from newly arrived E.U. citizens, who may not have the same rights? What will the cutoff date be? What will happen to welfare and pension rights, or the ability to bring over family members?

“After Brexit, our phones went off the hooks,” said Barbara Drozdowicz, director of the Eastern European Advice Center in London. “People phone in and ask, ‘Can I go home for Easter break? Will they still let me back in?’ ”

In a sign of how anxious many are feeling, E.U. citizens have created an advocacy group, called the 3 Million, to lobby for their rights.

“I think there is a legitimate sense of worry,” said Jonathan Portes, professor of economics and public policy at King’s College London.

“The government isn’t going to deport 3 million people, or even 1 million,” he said. “But equally, it’s not going to give a blanket guarantee. It’s not going to simply say to everyone who has an E.U. passport who is here today, ‘That’s it, that’s all you need, here’s your right to permanent residency and citizenship.’ ”

Britain is still a member of the E.U. — exit negotiations started Wednesday and are expected to last two years — and as such, ­freedom-of-movement rules still apply.

But some nervous Europeans are applying now for permanent residency, a necessary steppingstone on the path to citizenship. The hope for many is that the residency card will make it easier to grant them status post-Brexit.Although it isn’t mandatory — the card effectively acknowledges rights they already have — the number of E.U. citizens applying has doubled in the past year.

Applying is not easy. The form runs 85 pages — in Germany, a similar form is two pages. Extensive evidence, including pay slips, employment contracts and travel documentation, must be submitted. In the last half of 2016, more than a quarter of applicants who applied for permanent residency were rejected.

It’s not known what system the government will use in the post-Brexit settlement of E.U. citizens. If it were to use the current criteria for granting permanent residency, there would be an enormous backlog, and some of the people who have lived for decades in the U.K. wouldn’t qualify.

Last fall, Oxford University’s Migration Observatory calculated that it would take 140 years to process all of the E.U. nationals in the U.K. if the Home Office continued to churn through applications at its current rate.

There are also groups of residents who wouldn’t qualify under the current system. For instance, an estimated 470,000 people would need to show that they had “comprehensive sickness insurance,” or CSI. E.U. citizens residing in the U.K. are entitled to use the National Health Service, but if they are students or economically inactive, they also need to purchase insurance — a little-known requirement.

“This requirement was not known to anyone I know,” said Sabine von Toerne, a midwife who moved to London 13 years ago from Berlin. Speaking at the end of a busy day delivering babies as part of her work in Britain’s National Health Service, she said: “No one ever asked. Even when I enrolled into university for my midwifery degree, no one ever asked me, ‘Do you have CSI?’ ”

The 43-year-old, who has an 8-year-old British son, doesn’t think the authorities will deport her, but she worries that E.U. nationals could be treated as second-class citizens.

“I don’t think anyone will knock on my door and ask me to leave, that’s completely bonkers,” said Von Toerne. “What I could imagine is that there will be restrictions on health-care benefits and social security things.”

Mayall also had never heard of sickness insurance until after the Brexit vote, when she started looking into applying for permanent residency. She also didn’t save pay slips from her jobs in the late 1990s and early 2000s; at the time, she didn’t see any reason to keep them.

Mayall, who has a 12-year-old son and a 7-year-old daughter, recently saw an immigration lawyer who told her that her chances of securing permanent residency were weak.

“I’m too scared to fill out the application now. What if I’m rejected?” she said.

Her husband, James, 45, a veteran of the Royal Air Force, said he feels let down by the country that he served in various war zones. He even thought about giving back his medals. “I know it’s a sort of pointless gesture, but it’s how I thought,” he said.

He said that it’s “more than likely” the British government will come to an agreement about E.U. citizens, but at the moment he doesn’t have that reassurance. Under the current rules, he can’t sponsor his wife’s path to residency — British citizens can, however, sponsor spouses from non-E.U. countries like the United States.

“The worry is that the government will say, people who have lawfully lived in the country for ‘x’ amount of years can stay in the country. But if they use the word ‘lawfully,’ what does that mean for people like my wife who didn’t know about this insurance?”

“I find the whole situation deeply frustrating,” he said. “The British government thinks it’s perfectly acceptable to mess with people’s lives.”

Read more:

Britain’s next Brexit challenge: Retooling E.U. laws to fit into British system

Brexit critics say Theresa May is putting European security in danger

In historic break, Britain plunges into Brexit with hard negotiations still to come

Today's coverage from Post correspondents around the world

Like Washington Post World on Facebook and stay updated on foreign news