In its 2004 election manifesto the victorious Socialist party (PSOE) included plans to adopt a gender-blind succession rule like that used in Sweden, a proposal which was supported by the leader of the main opposition party, the conservative Partido Popular. It was initially thought that the change would only apply to future generations but with all the major political parties in agreement that the system of primogeniture conflicts with the constitutionally-established principle of sexual equality it is now planned that the law will be changed before the Princess of Asturias bears any male children, thereby securing the Infanta Leonor's position as second in line. The subsequent announcement, in 2006, that the Princess was pregnant with a second daughter has however removed any immediate urgency in the passage of the necessary legislation.
Section 57 of the Spanish Constitution provides that "The Crown of Spain shall be inherited by the successors of H. M. Juan Carlos I de Borbón." To date, the Spanish government has not seen fit to clarify whether this provision includes anyone beyond the descendants of King Juan Carlos. "Successor" is not the same as "descendant" but can be understood to mean such successors as brothers, sisters, uncles, aunts, cousins, etc. Presumably in the line of Borbón which surname is mentioned in the constitution. The king's two sisters renounced their rights of succession but those renunciations took place before the adoption of the Constitution and were not ratified by the Cortes as required by Section 57. The rights of earlier generations are similarly clouded by numerous renunciations and unapproved marriages which may or may not exclude the individuals involved from the throne.
Section 57 further provides that "Abdications and renunciations and any doubt in fact or in law that may arise in connection with the succession to the Crown shall be settled by an organic act." Presumably, legislation would be adopted to clarify the situation if all the descendants of Juan Carlos die off. As a final catch-all provision, Section 57 states that "Should all the lines designated by law become extinct, the Cortes Generales shall provide for succession to the Crown in the manner most suitable to the interests of Spain."
King Juan Carlos' three children are married and of child-bearing age. His daughters have already given him six grandchildren and his son, the Prince, has two daughters. As a result, it seems unlikely that Spain will need to look beyond the descendants of Juan Carlos to find future monarchs. Unless and until an organic act clarifies the rights of other members of the king's family, there is no way to determine who, if anyone, follows Infanta Cristina's descendants in the Line of Succession. As of , there are no plans to legislate on the question.