Predator Leonard Welsh was caged for nine years in 2004 for sickening sex attacks on two teenage girls.
Welsh, 63, died at HMP Edinburgh with a mandatory inquiry into a death in custody to start next month.
A Scottish Prison Service spokeswoman said: “Police Scotland have been advised and the matter will be reported to the Procurator Fiscal. A Fatal Accident Inquiry will be held in due course.”
Twisted Welsh met one of his victims in the street and invited her back to his then home in Logan Way, Livingston, in 1999.
The 15-year-old went to the house where Welsh took hold of her breast. When she tried to get free he seized her by the hair.
The girl screamed but he put his hand over her mouth, dragged her into a bedroom and told her: “Shut up and it will all be over and done with.”
Welsh then removed her lower clothes and raped her.
The attack came to light three years later after the victim was arrested for being found drunk and incapable.
She then revealed to police her earlier rape ordeal. Welsh earlier admitted raping the teenager in 1999.
He also admitted raping another 15-year-old on various occasions between June 1988 and May 1989 at a house in the Knightsridge area of Livingston.
The victim made it clear she did not want to have sex with him, but he threatened her.
A hearing at Edinburgh High Court in 2004 heard that she lost count of the number of times that Welsh raped her, but it may have been 25 to 30 occasions.
Lord Drummond Young told Welsh: “These offences involved threats of violence to the two unfortunate victims.
“It is clear a substantial custodial sentence is inevitable.”
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