Billie Eilish's ex-BF has Christian Bale to thank for their relationship not standing the test of time -- all 'cause of a random dream she had years earlier.

The singer spilled the beans about how a dream featuring "The Dark Knight" actor led her to end her relationship ... all while chatting about their favorite superheroes with Amelia Dimoldenberg at the Oscars Nominees' 'Pre-Luncheon' Luncheon alongside her bro, Finneas.

Billie recalls the dream -- it took place in a cozy little café, the sun shone in, and then ... bam, she wakes up ... and it's like a lightbulb goes off in her head! That's one hell of a wake-up call, literally!

She says it was at that very moment, still lying in bed after having the dream, that she knew she had to break up with her boyfriend.

The "Ocean Eyes' singer didn't reveal who she was referring to -- but it's worth noting that she's been linked to actor Matthew Tyler Vorce and rapper Brandon 'Q' Adams in the past.

Billie's most recent relationship was with The Neighbourhood's lead vocalist, Jesse Rutherford, but their love story hit a speed bump and ended last year. Her rep told TMZ at the time, "We can confirm Billie and Jesse did split amicably and remain good friends," ... while also shooting down cheating rumors that surfaced.

As for now, it looks like Billie ain't focused on no man ... 'cause her career's soaring to new heights! She and Finneas have their sights set on the Oscars Sunday, where they're nominated for Best Original Song for "What Was I Made For?" from "Barbie," and they'll also be performing at the ceremony.