The top destinations for British expats in the European Union are Spain (host to around 319,000), Ireland (249,000) and France (171,000).
Expats were able to vote in the EU Referendum (as long as they haven't lived abroad for more than 15 years).
Pro-EU advocates say that British expatriates residing in other European countries are able to thanks to the EEU's right of free movement, which means EU members cannot bar or expel citizens of other EU states.
On that basis, a former attorney general, Dominic Grieve QC, has argued that withdrawing from the EU would see British citizens living in EU countries "becoming illegal immigrants overnight" if Britain didn't maintain some form of free movement after officially leaving.
There have also been fears that member states angered by Brexit could try to put pressure on British expats in revenge. For example, Spain could ask British retirees to pay for their own healthcare, according to the Centre for European Reform's John Springford, or move to curb access to healthcare services outright.
In a paper outlining the risks of Brexit prior to the June 2016 vote, the Government said: "Many UK citizens would want any negotiations to secure their continued right to work, reside and own property in other EU states, and to access public services such as medical treatment in those states.
"UK citizens resident abroad, among them those who have retired to Spain, would not be able to assume that these rights will be guaranteed."
Could expats be barred from EU healthcare and benefits?
It's possible, but unlikely - not least given that it would open the door to retaliatory measures from the UK which hosts its own share of expats from European nations: there are as many as 3 million EU nationals living in Britain.
British expats can also claim to pay their own way in Europe, as the UK paid £674 million in 2014-2015 to other European countries for the treatment of UK nationals. However, the UK received just £49 million from other European nations in the same year to treat those from other countries residing in the UK.
Could expats be deported by EU members?
Almost certainly not. First, there are numerous political reasons for EU states not to do such a thing, including the treatment of their own, numerous, nationals living in the UK.
Mass expulsions of citizens from another developed economy would also startle foreign investors and potentially cause economic turmoil in the expelling country.
Expats would also enjoy significant legal protections that would apply after Brexit. Many lawyers argue that British expats living elsewhere in the EU at the time of Brexit would have individual "acquired rights" under international law.
This is based on the Vienna Convention of 1969, which says that the termination of a treaty "does not affect any right, obligation or legal situation of the parties created through the execution of the treaty prior to its termination.”
The House of Commons Library says that "withdrawing from a treaty releases the parties from any future obligations to each other, but does not affect any rights or obligations acquired under it before withdrawal."
In other words, Brits who have already exercised their right to live in EU states can expect to keep that right after Brexit. One important point though: this only applies to people who have started expat life in the EU before Brexit.
After Britain leaves, Brits’ ability to live and work in EU nations will depend on new agreements the UK negotiates with those nations.
David Cameron made this clear in his resignation statement, stating that there would be no "immediate changes" to expats' living circumstances.