The 1999 Royal Decree on foreign investments establishes the possibility to review and, if necessary, prohibit acquisitions of any asset by non-residents when these threaten Spain’s essential security interests.
"Article 2. Subjects of foreign investment.
1. They can be holders of foreign investments in Spain:
a) Individuals not resident in Spain, understood as Spaniards or foreigners, domiciled abroad or who have their main residence there.
b) Legal persons domiciled abroad, as well as public entities of foreign sovereignty.
2. Natural persons of Spanish nationality and legal persons domiciled in Spain shall be presumed to be resident in Spain unless proven otherwise.
[...]
Article 4. Declaration.
1. Foreign investments in Spain, and their liquidation, will be declared to the Investment Registry of the Ministry of Economy and Finance, for administrative, statistical or economic purposes.
2. The obligation to declare referred to in the previous section shall comply with the following rules:
a) If the purpose of the declaration is an investment that comes from tax havens, understood as the territories or countries provided for in Royal Decree 1080/1991, of July 5, the holder of the same must make it prior to the realization from the investment. This declaration will be understood without prejudice to the one to be made after the investment is made, in accordance with the following letter.
However, the following cases will be exempted from the prior declaration:
1. Investments in negotiable securities, whether issued or offered publicly, whether they are traded on an official secondary market or not, as well as participations in investment funds registered in the Registers of the National Securities Market Commission.
2. When the foreign participation does not exceed 50% of the capital of the Spanish company receiving the investment.
[...]
Article 10. Suspension of the liberalization regime.
1. The Council of Ministers, at the proposal of the Minister of Economy and Finance and, where appropriate, of the head of the competent Department by reason of the matter, and following a report from the Board of Foreign Investments, may agree, in a reasoned manner, with general or particular nature, the suspension of the liberalization regime established in this Royal Decree and provided that investments by their nature, form or conditions of realization, affect or may affect activities related, even if only occasionally, with the exercise of public power, or to activities that affect or may affect public order, public health and safety.
2. Once the liberalization regime is suspended, the affected investor must request prior administrative authorization regarding the investment operations that, from the moment of notification of the suspension, it intends to carry out.[...]" (Royal Decree on foreign investments (664/1999, of April 23), in force from 25 May 1999, Articles 2, 4 and 10 [google translation])
Royal Decree on foreign investments (664/1999, of April 23), in force from 25 May 1999, Articles 2, 4 and 10
[<a href="https://www.boe.es/buscar/act.php?id=BOE-A-1999-9938">Decree</a>]