"Lady in the Lake" killer Gordon Park has been found dead in his prison cell, a Prison Service spokesman said.
He is thought to have taken his own life.
Park was convicted of the murder of his wife Carol in 2005, nearly 30 years after he killed her and dumped her body in Coniston Water in the Lake District.
He was found hanging in his cell at HMP Garth, Leyland, Lancashire, this morning. It was his 66th birthday.
A Prison Service spokesman said: "HMP Garth prisoner Gordon Park was found unconscious in his cell at 8am on Monday January 25.
"Staff and paramedics attended but Mr Park was pronounced dead at 9.40am. As with all deaths in custody, the Prisons and Probation Ombudsman will conduct an investigation."
In the decades following the murder, Park cultivated an air of respectability and always maintained his innocence.
But while in prison on remand in October 1997, he admitted the killing, telling fellow inmates "she deserved it" after he discovered her in bed with another man.
Park was jailed for a minimum of 15 years and was not eligible to be considered for release until this month. Thursday is five years to the day since his conviction.
His son, Jeremy, who has campaigned to prove his father's innocence, said he was "devastated" by his father's death. He said: "We are all completely devastated and still believe his innocence 100%."