Mass murderer Charles Manson is set to marry a 26-year-old woman who has been trying to prove his innocence for the past decade.

Eighty-year-old Manson is serving a life sentence for the murder of seven people, including pregnant actress Sharon Tate.

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The Californian Department of Corrections and Rehabilitation confirmed Manson had been granted a licence to wed, but no date for the nuptials had been set.

Manson’s fiancee, Afton Elaine Burton, who goes by the name of Star, said she was completely in love with Manson and spoke to him every day.

Ms Burton said she had been following Manson’s white supremacist teachings since she was a teenager.

As Manson is serving a life sentence, his bride-to-be will only be able to visit him in prison, but said she was prepared to marry him under those conditions.

Sharon Tate’s sister, Debra, who acts as a spokeswoman for the families of Manson’s victims, said the impending marriage was ‘ludicrous’.

“I think it’s insane,” Ms Tate said.

“What would any young woman in her right mind want with an 80-year-old man?”

– with AAP