The only daughter of Spanish dictator Francisco Franco has died in Madrid aged 91, one of her grandchildren and her biographer said Friday.

Carmen Franco, who until the end fiercely defended her father's fascist legacy, had revealed earlier this year that she suffered from terminal cancer.

"God has taken back Man," her grandson Luis Alfonso de Borbon wrote beneath a picture of the elegant duchess on his Instagram account, using one of her many nicknames.

Her biographer Nieves Herrero meanwhile confirmed to El Mundo newspaper that "she died at her residence in Madrid".

The dark-haired mother of seven had led the National Francisco Franco Foundation, created in 1976, a year after the dictator died.

As recently as last year, when she came out of a memorial for her father, Franco supporters gave fascist salutes on the church square in the middle of Madrid.

"My father, it's up to history to judge him, not me," she said this year in the epilogue of a fictionalised biography written by Herrero based on conversations with her.

"When I am told that he was a dictator, I do not deny it but I do not like it because it is often said as an insult, whereas for me it does not seem so bad."

Carmen Franco, who was given the title of the "Duchess of Franco" by former king Juan Carlos I, was the mother of seven children and many grandchildren.

The family is regularly criticised in Spain for taking advantage of the dictator's fortune -- including a large amount of property -- that journalists have calculated runs into the hundreds of millions of euros.