BERLIN (Reuters) - Robert Harrison was enjoying a drink with a compatriot in a Munich beer hall a few weeks before Britain's referendum on leaving the European Union when he realised he and other Britons who stood to lose a lot of rights would have to get organised.
The Munich-based patent attorney had been in Germany for decades, living far from his country of birth with his mixed family and enjoying EU residence and employment rights that he, like many others, had believed irrevocable.
He joined local clubs, volunteered with refugees and even served as an election teller. "I was living the life of a German, never thought about being British," he said. "And then you suddenly have to organise with other Britons to campaign."
Britain's vote last year to leave pitched the million or so Britons living in the European Union into deep uncertainty.
Writing letters, using social media and lobbying politicians, expatriates have formed groups set on preserving their rights.
"It makes you think about the rights that you have taken for granted, pulled from under you," said Jane Golding, a Berlin-based lawyer who chairs British in Europe, a coalition that groups together the UK citizens' groups that have sprung up.
There was a big British community in Germany "long before the hipsters showed up in Berlin," said Alexander Clarkson of King's College London. "Suddenly, because of Brexit, British people feel their social position is under threat."
Britain and the EU this week held their first full round of talks on the exit process, but EU chief negotiator Michel Barnier said a "fundamental divergence" remained with Britain on how to protect expats' rights after Brexit.
Harrison recruited Britons to his cause through Facebook groups, campaigning first for a vote to stay in the EU, and, when that failed, for rights of British citizens.
This year, Mark Whiley, a software marketing specialist, used sponsored twitter adverts to attract 450 recruits to British in Berlin, whose members write to German politicians to raise awareness of their position.
Many of the groups have coalesced into British in Europe, which has member organisations across the continent. With European law specialists, marketing experts and business owners among their numbers, they are not short of skills.
The sheer range of individual circumstances defies easy collective solutions to the problems faced by Britons in the EU and the three million EU citizens living in Britain.
The uncertainty is forcing many to take major steps. Last year 2,865 Britons took German citizenship, a Brexit-driven 361 percent leap over the previous year. Yet even that does not fix all problems.
"Taking citizenship is one option, but a bigger issue for me is mutual recognition for qualifications and the rights of professionals to practise their profession," said Golding, a specialist in EU law.
Since such recognition is not automatic for qualifications from outside the EU, "citizenship is not a panacea for everything," she said.
For 150,000 retired Britons in Spain, one fear is that their pensions may suffer after Brexit, another that healthcare may no longer be available. Others fear that pension rights accrued in multiple countries will be lost.
Some two million British soldiers passed through Germany during seven decades during which Britain maintained large garrisons there after World War Two, and about 100,000 of them married Germans and stayed. Their status can be complex.
"I met a soldier who'd been there since 1955," said John Henderson, the retired general who shut down most of the bases in 2015. Born to a soldier on the base, he had signed up aged 18 and remained.
For such people, questions of attachment and emotional belonging are more complex than those faced by other expats.
With many feeling that citizens' rights have not so far been high on the British government's negotiating agenda, much of the focus has been on getting the attention of EU politicians such as Barnier or German Chancellor Angela Merkel.
Harrison, part-owner of a business that employs 25, jokingly asked Merkel at a recent Bavarian industry conference whether he could expect "dawn deportation" when Britain leaves. No, Merkel responded, advising him to consider taking German citizenship.
Spain is home to the largest number of British citizens in the EU, with 296,000 registered there - though the unofficial tally may be higher - meaning it is another hotbed of campaigning activity.
A big issue is the rights of families which include EU citizens, said one campaigner. "For instance, can I bring my Polish mother over to the UK to look after her if she gets ill?"
In Brussels, there is a particularly active British community. Brits Abroad Yes to Europe surveyed 6,000 affected people across Europe in January, using the results to lobby British and EU lawmakers.
Golding reckons British in Europe numbers around 30,000 people, and, collaborating with The Three Million, a similar grassroots group representing EU citizens in Britain, has succeeded in winning the attention of British authorities.
She and her colleagues have five times been invited to consultations with the British government as well as to see the European Commission's negotiators.
"The coalition is growing fast as new groups in new countries join," said Golding, who lived in three European countries before moving to Berlin with her German partner. "It's unprecedented for UK citizens' groups to come together and work in a coalition like this."
Reporting by Thomas Escritt in Berlin, Sonya Dowsett in Madrid and Elizabeth Miles in Brussels; editing by Giles Elgood