Prince Pedro, Duke of Calabria

On 14 December 1900, Prince Carlos, next oldest brother to the childless Prince Ferdinand, head of the House of Bourbon-Two Sicilies and irrefutable claimant to the extinct throne of the Two Sicilies, signed a private agreement of renunciation to the disappeared throne as he was marrying María de las Mercedes, Princess of Asturias, heir presumptive to the throne of Spain.[4] This document, known as the Act of Cannes, was signed in purported obedience to the 1759 Pragmatic Sanction signed by Charles III of Spain where it was established that the thrones of Spain and Naples should never be held by the same monarch and thus divided them for the sake of a European balance of power.[5] The newly independent Kingdom of Naples was ceded by Charles of Spain to his third child, who would become Ferdinand I of Naples.[5] This would establish the kings of Naples and Sicily as cadet members of the Spanish royal family, and so the country enjoyed strong relationships with its 'mother state', following many of its legal customs. The Act of Cannes states:

Before Us, Don Alfonso de Borbón, Count of Caserta... Head of the Royal House and Dynasty of the Two Sicilies... His Royal Highness Prince Don Carlos, our beloved Son, appears and declares that, having to marry HRH Infanta María de las Mercedes, Princess of Asturias, and assuming by such marriage the nationality and quality of Spanish Prince, understands to renounce as by this Act solemnly renounces, by himself and by his heirs and successors, all the right and reason to the eventual succession to the Crown of the Two Sicilies and to all the assets of the Royal House that are in Italy and elsewhere, and this according to our Laws, constitutions and Family customs, in execution of the Pragmatic Sanction of King Charles III, our Augustus ancestor, of October 6, 1759, the prescriptions of which he freely and spontaneously declares to subscribe and obey. He also declares, in particular, to renounce for himself, his heirs and successors to the assets and values existing in Italy, Vienna and Monaco (Bavaria) and destined by His Majesty King Francis II (may God have welcomed his soul), to the foundation of a majorat for the Head of the Dynasty and of the Family of the Two Sicilies and for the constitution of an endowment fund in favor of the Royal Princesses and granddaughters of our August Father King Ferdinand (may God have welcomed his soul), of marriageable age; but preserving his rights to the part of the assets that were bequeathed to him by his late uncle King Francis II, in the event that the Italian Government, which improperly retains them, makes the due restitution and the same everything that may arrive to him by other testamentary legacies.

— Cannes, 14 December 1900[4]

Supporters of the other claimant to the headship of the House of Bourbon-Two Sicilies, sometimes referred to as the Castrist faction, argue that because Prince Carlos signed this agreement, he relinquished all of his rights and those of his descendants to both the headship of the family and the Sacred Military Constantinian Order of Saint George, and so the rights currently fall on Prince Carlo, Duke of Castro. However, supporters of Pedro, also known as the legitimists, argue that the Act of Cannes was drafted upon conditions that never arose, i.e. it would have only been valid if Prince Carlos would have indeed faced the situation of his wife inheriting the throne of Spain, something that never happened and was furthermore highly unlikely at the time the document was created. The legitimist supporters also point out several flaws in the document and its interpretation. Also, they argue that the Act of Cannes never mentioned the headship of the House of Bourbon-Two Sicilies or the grand magistry of the Constantinian Order, and so this was therefore never renounced. Regardless, it was founded as trustee Farnesian dynastic order, and so it is the mainstream academic view that succession is not linked to the throne of Naples and Sicily. Following this logic, at the very least, the headship and grand magistry of the Constantinian Order would fall on Prince Pedro.[6]