The 48-year-old recorded two songs ten years ago and distributed them to only a handful of her closest friends in the village of Blackburn, West Lothian.
The recording, which was unearthed by Telegraph.co.uk, features two songs - an early version of the blues ballad Cry Me a River and Killing Me Softly with His Song, the track immortalised by Roberta Flack in 1973.
The recording was made at her own expense with Miss Boyle telling friends at the time that she had "spent all her savings" on it.
A friend of Ms Boyle, who gave the recording to Telegraph.co.uk, said she had forgotten about the tape until her friend shot to global fame.
The friend, who did not want to be named, said: "I had heard lots of stories of what a great singer Susan was but I had never heard her sing.
"So she came past one day and told me she had made this tape of a couple of songs – she said to take care of it as it had cost her all her savings to get it made.
"She said she hadn't had many made and had only given a few out to her closest friends in the village.
"I forgot about it then when I watched Britain's Got Talent I suddenly remembered I had it. I listened to it again and thought it was incredible that she had not been discovered until now.
"Now millions of people all over the world have fallen in love with her voice and I hope they get to hear this tape. It's an incredible story."
The recording, which was unearthed by Telegraph.co.uk, features two songs - an early version of the blues ballad Cry Me a River and Killing Me Softly with His Song, the track immortalised by Roberta Flack in 1973.
The recording was made at her own expense with Miss Boyle telling friends at the time that she had "spent all her savings" on it.
A friend of Ms Boyle, who gave the recording to Telegraph.co.uk, said she had forgotten about the tape until her friend shot to global fame.
The friend, who did not want to be named, said: "I had heard lots of stories of what a great singer Susan was but I had never heard her sing.
"So she came past one day and told me she had made this tape of a couple of songs – she said to take care of it as it had cost her all her savings to get it made.
"She said she hadn't had many made and had only given a few out to her closest friends in the village.
"I forgot about it then when I watched Britain's Got Talent I suddenly remembered I had it. I listened to it again and thought it was incredible that she had not been discovered until now.
"Now millions of people all over the world have fallen in love with her voice and I hope they get to hear this tape. It's an incredible story."
