Britain’s Foreign Secretary, Philip Hammond, has said the immigration status of British expats in EU countries could be in doubt if the referendum decides that the UK should leave the EU.
Mr Hammond said it is not clear what rules would apply if the UK was no longer part of the EU's free movement rules.
The impact could be felt by hundreds of thousands of expats who might have to return to the UK. It would seemingly be felt by the EU nationals currently residing in the UK.
He said that Britain and Spain might have to agree quotas of Britons and Spaniards who can live in their respective countries.
Agreements could also be needed for other EU nations.
After speaking at a Chatham House foreign affairs think tank, Mr Hammond, who is in favour of staying in the EU, was asked what Brexit would mean for EU immigrants in the UK. He said: "It's a very good question and, again, a question that the Leave campaign has to answer.
"There are two million British citizens living in European Union countries.
"We have spent a lot of time in the UK focusing on EU citizens living here, but we have two million of our own citizens living in the EU - the largest number living in Spain.
"Somebody has to answer that question. Would we negotiate bilateral agreements, would there be quotas?
"What do the Leave campaign have in mind? If they are going to impose quotas, would they be retrospective? Would they expect them to be reciprocal?"
He went on to say that there would be at least two years for the UK to negotiate an exit from the EU, as mandated by treaties. And that this was likely to be a much longer period to make all the trade agreements needed.
