The Theravada accepts the Jataka stories as commentarial literature; they are not canonical, but are considered a reliable account by a learned Buddhist scholar. What are canonical are the verses that accompany the stories ...
It's not uncommon for modern Theravada Buddhists to question the authenticity of the stories, especially given their often fantastical content.
Based on the above, it looks like the stories of the Buddha's past lives from Jataka are merely commentarial, and not canonical.
However, I found one possible story of the Buddha's past life as told by him in MN 83, which is part of the Sutta Pitaka, but outside Jataka. Being part of the Majjhima Nikaya, I take it that this is canonical.
It's a story about the just and principled King Makhadeva who started the practice of retiring from the throne at an advanced age to become an ascetic. He practiced the Brahmaviharas, and was reborn after the break-up of the body, after death, in the Brahma realm. He established and passed on this good practice to his descendents who carried on doing it, till King Nimi's son Kaḷārajanaka who stopped this practice.
The plot twist is when the Buddha tells Ananda:
And having developed the four Brahmā meditations, when his body broke up, after death, King Nimi was reborn in a good place, a Brahmā realm. But King Nimi had a son named Kaḷārajanaka. He didn’t go forth from the lay life to homelessness. He broke that good practice. He was their final man.
Ānanda, you might think: ‘Surely King Makhādeva, by whom that good practice was founded, must have been someone else at that time?’ But you should not see it like this. I myself was King Makhādeva at that time. I was the one who founded that good practice, which was kept up by those who came after. But that good practice doesn’t lead to disillusionment, fading away, cessation, peace, insight, awakening, and extinguishment. It only leads as far as rebirth in the Brahmā realm. But now I have founded a good practice that does lead to disillusionment, fading away, cessation, peace, insight, awakening, and extinguishment. And what is that good practice? It is simply this noble eightfold path, that is: right view, right thought, right speech, right action, right livelihood, right effort, right mindfulness, and right immersion. This is the good practice I have now founded that leads to disillusionment, fading away, cessation, peace, insight, awakening, and extinguishment. Ānanda, I say to you: ‘You all should keep up this good practice that I have founded. Do not be my final men.’ Whatever generation is current when such good practice is broken, he is their final man. Ānanda, I say to you: ‘You all should keep up this good practice that I have founded. Do not be my final men.’”
The Pali version side-by-side with English:
I myself was King Makhādeva at that time.
Ahaṃ tena samayena rājā maghadevo ahosiṃ.
I was the one who founded that good practice,
Ahaṃ taṃ kalyāṇaṃ vattaṃ nihiniṃ, mayā taṃ kalyāṇa vattaṃ nihitaṃ;
Is my interpretation correct that the Buddha was King Makhadeva in his past life, based on the Pali version?
Are there other such canonical stories of the Buddha's past lives in the Pali Sutta Pitaka, outside of Jataka?