The Maggie May hitmaker reveals he was left heartbroken after his third wife walked out on him during a difficult period in their marriage, and they were separated for two weeks.
The couple, who wed in 2007, eventually reconciled, but Stewart used the agonising ordeal as inspiration for a song on his latest album.
He explains, "I cover Tom Waits' Picture In A Frame on my new album Time. Penny and I broke up for two weeks and the song is about that time. I asked her for a photo of herself in a frame so that I could see her when I went to bed and when I woke up."
Stewart previously had a reputation as a womaniser but he is adamant Lancaster is the one for him, telling Australian magazine Life, "I have always loved women, won some, lost some, but it's not about the magic touch, it's about being a better person and a good listener. I met Penny in my 50s and if I had not learnt by that point in my life I was never going to."