In an "Alternative White Paper" timed to coincide with the British government's launch of its blueprint for Brexit, organisations representing British citizens living in Spain, Germany, France, Belgium, and Italy called for their rights as EU citizens to be preserved after the pull-out.
"The (Brexit) referendum gave no mandate to alter the rights of these people," read the document drafted by Jane Golding and Jeremy Morgan, two British lawyers living in Germany and Italy respectively, and endorsed by 10 groups, including Bremain in Spain.
"It was no part of the (pro-Brexit) Leave campaign that their rights should be torn up, quite the contrary.
"It is therefore essential that whatever steps are necessary to protect these rights are taken, and taken as a matter of urgency to bring an end to the anxiety that they are feeling about their personal futures and those of their families."
The document called for expats' pre-Brexit rights to be included in EU withdrawal negotiations and explicitly guaranteed in a final agreement "so as to give it the force of international law."
It is estimated at least 1.2 million British citizens live in the 27 other EU member states, though that number could be far higher.
Chief on their minds are healthcare, pensions, the right to work, to study, and to generally remain in their country of residence - all of which are addressed in the "alternative white paper."
In Spain, for instance, which counts a huge British community, many of them pensioners who have moved to better climes, healthcare is a huge concern, particularly with the fall in the pound following June's referendum.
For now, it is covered by Spain's social security system under EU mechanisms that allow Britons to receive the same treatment as locals, and many would not be able to pay if that right were taken away.
The "alternative white paper" also highlighted the case of professionals such as nurses or architects whose qualifications are recognised EU-wide, and would have to re-qualify in their country of residence.
"It's very important to speak with one voice," Golding, a member of Germany's "Brits in Europe" association, told AFP.
"These are real people, real lives, real problems we're facing."