If Britain votes on 23 June to leave the European Union I, and an estimated 1.3 million other so called expats living in EU countries, could find ourselves becoming illegal immigrants unless the UK government maintains some form of free movement agreement with the bloc it has exited.

I have lived in Madrid since 1986, which – adding insult to potential injury – disqualifies me from voting in the referendum as only those who have lived for less than 15 years abroad can do so. A group of Britons who have lived abroad for longer recently presented a legal challenge against the UK government in the High Court in London on the grounds that they have been wrongly disenfranchised. These individuals say they are “Britons in Europe” using their EU freedom movement of rights to live and work in Europe and “are not expats.” I, too, regard, myself as a Briton in Europe.

The top destination in the EU for Britons living outside their own country is Spain (more than 300,000), many of them retirees. As a result of Brexit, we could find our continued right to work, reside and own property in Spain and to access public services such as medical treatment is no longer automatically guaranteed. I have paid into both the British and the Spanish Social Security systems, unlike those retirees who paid into just the British system, so health care, at least, should not be denied to me.

There is considerable uncertainty over what will happen to us. Will the fact that we lived in Spain (or other EU countries) at the time of Brexit mean that we would have obtained “acquired rights” under international law? In other words, those who have already exercised their right to live in EU states BEFORE Brexit would keep that right AFTER Brexit. According to the library of the British parliament, which provides impartial information, “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.” But EU countries, angered by Britain exiting, could decide to be vindictive and put pressure on us by, for example, making foreign homeowners pay more tax.

My decision to leave the UK at the age of 35 was a very conscious one. I hated living on an island and the “little Englander” attitude. Having covered Spain’s transition to democracy as a correspondent of The Times (1975-78), the country fascinated me (in the tradition of the curiosos impertinentes). My wife and I bought a ruin of a house in a village in the province of Cuenca in 1976, which became a microcosm of the profound changes taking place and which we still have. When I moved from Spain to Mexico for the Financial Times (1978-84) and then to London (1984-86) I felt like a foreigner in my own country. By then my wife and I had adopted two Mexican boys. Our desire for them to be bi-lingual (much more so than their adopted parents) was one of many reasons for returning permanently to Madrid in 1986, which we were able to do thanks to both Spain and the UK being members of the EU, a marvellous concept whatever its defects. In the words of the English expression, home is where the heart is. The EU allowed me to achieve this.

Now the situation could change, and even if it does not, I have decided that if Britain leaves the EU I will seek Spanish nationality in order to turn my back completely on my country of birth and in recognition of all that Spain has done for me professionally, personally and emotionally.

For very very different reasons, I would follow the example of the writer Arturo Barea, one of my heroes whose commemorative stone in the cemetery of the town of Faringdon in the county of Oxford I helped to restore. Barea fled the Spanish civil war and spent the last 18 years of his life in England. He loved the country of his exile, just as I love mine, though I do not consider myself to be in exile, and became a British citizen in 1948. Perhaps I will not be alone in seeking to change my nationality.