Constance of Castile died in 1345. Afonso IV tried several times to arrange for his son to be remarried, but Pedro refused to take a wife other than Inês, who was not deemed eligible to be queen. Peter's legitimate son, future King Ferdinand I of Portugal, was a frail child, whereas Peter and Inês's illegitimate children were thriving; this created even more discomfort among the Portuguese nobles, who feared the increasing Castilian influence over Peter. Afonso IV banished Inês from the court after Constance's death, but Peter remained with her declaring her as his true love. After several attempts to keep the lovers apart, Afonso IV ordered Inês's death. Pêro Coelho, Álvaro Gonçalves, and Diogo Lopes Pacheco went to the Monastery of Santa Clara-a-Velha in Coimbra, where Inês was detained, and killed her, decapitating her in front of her small child. When Peter heard of this he sought out the killers and managed to capture two of them in 1361. He executed them publicly, ripping their hearts out claiming they didn't have one having pulverized his own heart.
Peter became king of Portugal in 1357 (Peter I of Portugal). He then stated that he had secretly married Inês, who was consequently the lawful queen, although his word was, and still is, the only proof of the marriage. During the 1383–85 Crisis of royal succession in Portugal, João das Regras produced evidence that allegedly established that Pope Innocent IV had refused Peter's request to recognize his marriage to Inês and legitimize his children by her, the elder of whom, John, Duke of Valencia de Campos would have a strong potential claim to the throne of Portugal. By negating these children's claimed legitimacy, João das Regras strengthened the claim of another illegitimate child of Peter I of Portugal: John, Master of Aviz, who ultimately took the throne and ruled as John I of Portugal.
Some sources say that after Peter became king of Portugal, he had Inês' body exhumed from her grave and forced the entire court to swear allegiance to their new queen: "The king [Peter] caused the body of his beloved Inês to be disinterred, and placed on a throne, adorned with the diadem and royal robes. and required all the nobility of the kingdom to approach and kiss the hem of her garment, rendering her when dead that homage which she had not received in her life..." Some modern sources characterize the story of the Ines' post-mortem coronation is a "legend." She was later buried at the Monastery of Alcobaça where her coffin can still be seen, opposite Peter's so that, according to the legend, at the Last Judgment Peter and Inês can look at each other as they rise from their graves. Both marble coffins are exquisitely sculpted with scenes from their lives and a promise by Peter that they would be together até ao fim do mundo (until the end of the world).
Inês de Castro and Peter I had several children, who were legitimized by Peter I on 19 March 1361: