Fritzl, 73, wanted to charge visitors to enter his former home in the town of Amstetten in a bid to raise money for his traumatised family.

His sister-in-law, named only as Christine R, revealed his plans in an interview with the Austrian tabloid Oesterreich on Saturday. She said: “He actually wanted to make a tourist attraction out of his house with the dungeon where he kept Elisabeth, and charge ten euros for entrance.

“It is completely mad. The family was supposed to get the proceeds, but, naturally, all of them rejected his ‘business’ proposal.”

He has confessed to imprisoning and sexually abusing his daughter Elisabeth, 42, in a rat-infested, windowless subterranean bunker beneath his home, where she gave birth to seven of his children.

Last week, he was charged with murdering one of the children he fathered by his daughter. Prosecutors say he is responsible for his son’s death as he failed to get proper medical attention when the baby fell ill shortly after birth.

A physician specialising in newborns commissioned by the prosecution found that the child had symptoms, including breathing difficulties and swellings on his body, and that Fritzl must have recognised that the baby’s life was in danger.

The 73-year-old retired electrician has also been charged with rape, incest, false imprisonment and slavery.

The charges against Fritzl, which were leaked to Austrian press, revealed previously unknown details about the ordeal his captive family was subjected to in the dungeon. According to prosecutors, he would turn off the electricity in the subterranean prison, leaving his children in complete darkness for hours, as punitive measure when they would not behave the way he wanted them to.

The trial is expected to begin in March 2009, and Fritzl will be facing a life in prison for the murder charges.

His daughter Elisabeth and her six surviving children, aged five to 19, are living together at a secret location and are said to be recovering with the aid of psychological counselling provided by the state.