The Led Zeppelin legend was rumoured to have married the singer in secret in 2013 after setting up home together in Texas and collaborating on a number of projects, but Griffin denied the speculation saying, "We're not legally married, no."
However, Plant has now revealed the relationship came to an end, telling Britain's Radar magazine he experienced a shift both "culturally and slightly spiritually" and ploughed his heartbreak into his new music.
He adds of a recent trip back to his native U.K. with Griffin, "She didn't share my penchant for cider and the Black Country (English area) character I became after four pints of Thatchers (cider). My feelings are very much ones of sadness and regret."
The publication reports Plant's song House of Love, from his upcoming album Lullaby and... The Ceaseless Roar, is about the break-up and starts with the line "Leaving was so hard."