On March 26, 2019, Spain's Official State Gazette published the Instructions establishing the documentation procedure that will be followed if a non-deal Brexit takes place, which shall be applicable to UK citizens and their family members who were residents in Spain prior to the withdrawal and to UK citizens who were cross-border workers prior to the withdrawal.

In terms of immigration, when the United Kingdom departs from the European Union, UK citizens will become citizens of a non-member state. These UK nationals and their families will no longer be eligible for the EU regime but will be subject to the general immigration regime.

In preparation for the foregoing, the Instructions establish a 21-month period from the date of the potential non-deal Brexit for such individuals to substitute or apply for new documents, as explained below:  

UK citizens who already hold a certificate of EU registration may apply for its automatic substitution; changing their certificate of registration for what is known as the "TIE" (Identification Card for Foreigners / Tarjeta de Identidad de Extranjeros). The TIE for UK nationals will be valid for a period of up to five years, depending on the time left to reach such maximum period, based on the date that their registration certificate was issued.  
Relatives of UK citizens who hold residence permit cards as EU national family members will have to apply for automatic substitutions of these current cards for the aforementioned TIEs. With regard to such relatives, their cards will have the same expiry date as the UK citizen to whom they are related.  
Regarding UK citizens and family members who have legally resided in Spain for an uninterrupted period of at least five years, the Instructions establish that they will be eligible for a long-term residence card (permanent residence).  
For UK citizens and family members who do not have registration certificates or identification cards as EU national family members but who were residents in Spain before the withdrawal date, the Instructions establish a procedure to authorize a residence and work permit as long as they meet conditions similar to those that are currently applied to document EU country nationals. This residence/work permit is subject to approval by the relevant immigration authority.  
Finally, the Instructions define "cross-border workers" as those who carry out business or professional activities as self-employed workers or those who are employees of a company or a third-party employer in the territory of Spain, but reside in another Member State or territory to which they return once a week, at a minimum. In such cases, the procedure for requesting and substituting the current EU citizen registration certificates will not be automatic, given that said individuals will have to apply for a cross-border worker's work and residence permit. In these cases, in contrast to those mentioned above, the documentation will be valid for a year, with potential one-year extensions. 

The foregoing is established notwithstanding the fact that UK citizens and their family members affected by this potential no-deal withdrawal of the United Kingdom from the European Union could also apply for any of the authorizations or work permits established under Spain's immigration legislation/regulations that are applicable to non-EU nationals, in accordance with the requirements for each type, in the understanding that this option may actually be more beneficial than what is established in the contingency measures. For example, a Residence Permit for Investors may be a better option for the applicant in some cases.

Finally, the Instructions indicate that any administrative procedures that were initiated before the Brexit takes place will be processed in accordance with these Instructions.