Important for many if not most who have kept hold of their British passports or continue to use their original nationality.
Spaniards who are not of origin (for example, those who have acquired Spanish nationality by residence) lose their nationality if:
After acquiring the Spanish nationality the citizens continues to use during a period of three years the nationality to which they had renounced when acquiring Spanish.
When they voluntarily enter the service or exercise political office in a foreign State against the express prohibition of the government.
When it is considered that the party has committed fraud or concealment of data when making the application for acquisition of Spanish nationality.
* In the case of Spaniards who were born abroad of a Spanish parent also born abroad, will lose the Spanish nationality if within three years from the emancipation or coming of age do not declare His/her will to preserve Spanish nationality.
How to recuperate Spanish nationality
How to recuperate at consulates