Queen Sofia became emotional during a public engagement in Madrid on Friday.
King Felipe's mother, 84, broke down in tears as she paid tribute to her former university professor and longtime friend Emilio Lora-Tamayo in a speech.
The physicist was appointed honorary rector of the Camilo Jose Cela University at the ceremony.
Queen Sofia's display of emotion comes just days before her granddaughter Princess Leonor's swearing-in ceremony at Congress on her 18th birthday on Tuesday.
However, the former queen will reportedly miss the historic ceremony at the Cortes Generals.
Sofia's estranged husband, Spain's former King Juan Carlos I, will also be absent from the event, due to concerns that his presence may overshadow his granddaughter's milestone moment.
Juan Carlos abdicated in favour of his son Felipe in 2014, and since 2020 he has lived in self-imposed exile in Abu Dhabi, after a string of scandals.
After the swearing-in ceremony, the Spanish royals will then gather at the Royal Palace of Madrid, where Leonor will receive the prestigious necklace of the Order of Charles III.
The necklace, created by Carlos III in 1771, represents that Princess Leonor will one day become queen.
Juan Carlos I and Sofia will reportedly join the royals at a private family lunch after the ceremony.
The royal sisters joined their parents for a series of public engagements in the principality of Asturias in northwest Spain last week.
Queen Sofia also joined her son, daughter-in-law and granddaughters at the annual Princess of Asturias awards, where Leonor gave a speech.
If Leonor ascends to the throne as expected, she will be Spain's first queen regnant since her fourth great-grandmother, Isabella II, who reigned from 1833 to 1868.
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