A question we have seen people asking is something along the lines of the following:-
“Do the Spanish Government allow Spanish citizens to hold dual nationality with, for example, the UK ? It is possible for someone to hold a Spanish passport and a valid UK passport at the same time? Just worrying about Brexit and rights of UK citizens afterwards in Europe.”
Spanish nationality can be acquired through the rights of residence, if you have lived enough years in Spain continuously before you make an application to become a Spanish national, which for most people is 10 years.
You can make the application if you are over 18 years old (see this link here (http://www.mjusticia.gob.es/cs/Satellite/Portal/en/ciudadanos/tramites-gestiones-personales/nacionalidad-residencia) and you will need a certain amount of paperwork. This includes a birth certificate translated, criminal record certificate and also you are required to take a Spanish language test and a test on constitutional and socio cultural knowledge of Spain. The application costs 101 euros.
Although it may seem attractive to some people who have been Resident here a long time to become a Spanish citizen, you have to take into account that the contract you sign applying for Spanish nationality asks you to renounce all previous nationalities from other countries (bar a few exceptions).
Spain does not have an agreement for dual nationality with the UK, so you do not have a choice to keep dual nationality. You cannot.
This means you would be giving up your British passport (although the British government may not ask you to hand it in), and also once you are a Spanish national you will need to abide by the Spanish laws regarding inheritance, so you will be forced to follow the Spanish laws on succession and forced heirs.
It also affects your rights to live in the UK and if you went back you would have to follow immigration rules.
Thus this is a very serious decision to take and not one many people should consider lightly.