According to United Nations statistics, there were 1.22 million Britons living in other EU countries at the end of 2015, including more than 300,000 in Spain, 185,000 in France and 100,000 in Germany.
All of them face considerable uncertainty if the UK backs Brexit - both financial and, even more fundamentally, question marks over their right to continue living in Europe.
Foreign exchange analysts caution that anyone living overseas who receives income in sterling - from an employer in the UK, say, or from UK state or private pensions - could be significantly worse off under Brexit, as the pound comes under pressure from international investors withdrawing.
Healthcare will be a major worry for many. Under current reciprocal EU agreements, Britons travelling or living in other member states are generally entitled to free healthcare. That will no longer automatically be the case if the UK leaves the EU.
Property & Mortgages
While there’s little chance of the Union deciding all expats must be shipped back to their homeland, the countries in it could make things trickier for Brits if they feel so inclined.
Expats who own property in popular countries such as Spain and France have already seen valuations fall sharply in recent years, courtesy of economic uncertainty fuelled by Brexit.
It’s not all bad news, a new EU regulation on mortgage lenders which makes lending to expats (i.e. offering foreign currency mortgages) more complex have led to a number of lenders pulling out of the expat market.. If the UK was to leave the union, lenders may be more willing to offer loans to Brits abroad.
If Brexit does take place, expats are going to have to study the detail of the subsequent negotiations very carefully to find out how they're affected.