Born Infante of Spain, as third son of King Alfonso XIII and Queen Victoria Eugenia (born HH Princess Victoria Eugenia of Battenberg, a British Princess, later created HRH before her marriage). In 1933, after the renunciations of his elder brothers, he became the heir to the Spanish throne, although the monarchy had been abolished in 1931, and became the Prince of Asturias.
After his father's abdication and death, he became the Head of the Spanish Royal House and Family and adopted the title of Count of Barcelona, one of the titles of sovereignty of the Kings of Spain, tite recognised by the Spanish State, that had restored the monarchical system (although with no King) in 1947.
He renounced his rights and royal titles in 1977, after the proclamation of his son Juan Carlos as King of Spain in 1975, but was allowed to continue to use the royal title of Count of Barcelona. Since 1987, he had the honours of Prince of Asturias. But, after his death, the King issued a Royal Decree, granting him the honours of King of Spain and authorised his remains to be buried with the Kings of Spain. His remains are still at the Pudridero of El Escorial, but the inscription on his urn can already be seen. It says: "Juan III, Count of Barcelona", in latin.
Next Tuesday, the King will preside over a Mass at the Basilica of El Escorial. King Juan Carlos and Queen Sofia will be present, as well as other members of the family, although no list has been published.