London - Ending free movement of people in order to cut down on the number of EU citizens moving to Britain was one of the main drivers behind the Brexit campaign.
The key question for the government is what will happen to the more than 3.6 million non-British European Union nationals already living in Britain once the country leaves the EU in two years' time.
Prime Minister Theresa May has said this will be a priority for the negotiations and has linked the issue to the future of the estimated 1.2 million Britons living in the EU's 27 other member states.
The European Commission is also making the fate of EU citizens a priority but says the issue is complex as it also affects pensions and social welfare payments.
EU nationals are following the debate with growing concern.
How many Europeans live in Britain?
Some 3.6 million European Union citizens were living in Britain in 2016, of whom around a third were in London, according to the University of Oxford's Migration Observatory, which uses the latest data from the Office for National Statistics (ONS).
Estimating the number of foreigners is a headache for the government since there is no obligation to register with their respective European consulates.
France, Germany and Spain estimate the numbers of their citizens living in Britain at around 300,000 - around double the official estimates.
At one million citizens, Poland is by far the European Union member state with the highest number of its citizens living in Britain.
In 2015, Polish-born UK residents overtook Indian-born as the biggest minority in Britain.
The influx is partly due to Britain being one of the member states that completely opened up its labour market to Poles when Poland joined the EU in 2004.
The lifting of restrictions in January 2014 for Bulgarians and Romanians had a similar effect.
The increase in arrivals was particularly high from Romania, which is now the third biggest provider of Europeans to Britain after Poland and Ireland.
These successive waves of immigration from eastern Europe have caused resentment in parts of Britain where they have transformed the demographics.
These tensions came to the fore in last year's Brexit referendum in which 52 percent voted to leave the EU.
Net migration - the difference between the number of arrivals and departures - reached a record high in November 2015, seven months before the referendum.
Official data published in February showed the net figure at 273,000 in the 12 months to September 2016 - its lowest level since June 2014.
The ONS said this decrease, hailed by the British government, was in part explained by increased departures of east Europeans since the referendum.
But Nicola White from the ONS, said it was "too early to say what effect he referendum result has had on long-term international migration".
Why do Europeans move to Britain?
All the studies indicate that European immigrants come to Britain primarily to work.
Out of the 3.6 million EU citizens living in Britain, 2.2 million are employed, the ONS said.
In 2015, 73 percent of the new arrivals, attracted by a dynamic labour market and low unemployment, were coming to work and 21 percent to study.
Europeans work mainly in hospitality and tourism (24 percent), finance (18 percent) and public services such as the National Health Service (17 percent).
These sectors are among the main ones worried about the consequences of Brexit, particularly healthcare.
A parliamentary committee report in April said that 60,000 EU citizens work for the NHS in Britain.
"The impact of Brexit on the morale (of EU staff) is concerning and the uncertainty is unwelcome," the report said.
In the City of London, banks are feeling the push to move some of their activities to continental Europe.
In farming, which employs a lot of foreign seasonal workers, the proposed end of free movement is also causing a lot of concern.
"We don't have a business model without them," said Nick Ottewell, a salad grower in Kent.
In the run-up to the referendum, Britain's Conservative government proposed limiting access to some social welfare for EU citizens to win over Britons to remain in the EU and ward off eurosceptics.
Brexit supporters said the new arrivals are also adding pressure on housing, schools and hospitals.
Some have also blamed EU citizens for lowering salaries for British workers, although this charge is contested.
"The short answer is 'no'. It doesn't have any effect on wages at all," said Jonathan Wadsworth, author of a report on the subject at the London School of Economics.
What future for Europeans?
Britain and the other 27 countries have said they want the fate of EU citizens in Britain and British citizens in the EU to be a priority.
But this has done little to lower concerns.
Will people need a visa? A work permit? Will there be national preferences? A points-based system or a minimum salary requirement? Is it time to leave?
Requests for British citizenship and permanent residency rights have increased sharply since the referendum result.
This has been described as a "bureaucratic nightmare" by applicants who can apply for residency rights after living in Britain for five years or more.
The British government has recently eased the red tape required but it remains burdensome.
The documentation required weighs in at 10 kilos, according to Nicolas Hatton, a Frenchman who has lived in Britain for two decades and is the founder of the3million, a campaign group for EU citizens.
A Spanish nurse working in the state-run National Health Service (NHS), Joan Pons, has been living in Britain for 17 years.
Since the Brexit vote, this father of three children who were all born in Britain, said they are "scared that we'll go on holiday and never come back".
"Brexiteers" have said these fears are unfounded and assured those already here that they can stay.
But what about the new arrivals? What will be the cut-off date for who can stay? How will they organise their professional and family lives?
For low-skilled workers in the catering industry, the British government is looking into the possibility of a temporary travel permit nicknamed a "barista visa" which would be limited to two years and would give no access to social welfare but would allow EU citizens to work in Britain's many coffee shop chains.
Similar schemes would apply to seasonal farm workers.
Nico, a 25-year-old Romanian working on a salad farm in Kent, said he was not concerned.
"The farms need us so I'm not worried," he said.
But other European migrants have chosen to leave the country and British hospitals are struggling to recruit.
According to data from the British Nursing and Midwifery Council published in January the number of European nurses applying to join the NHS has fallen by 90 percent since the referendum.
Janet Davies, chief executive of the Royal College of Nursing, warned that the NHS could suffer a further crisis if the trend continued.
"With 24,000 nursing vacancies across the UK, the NHS simply could not cope without the contribution from EU nurses," she said.
By Jacques Klopp