Josef Fritzl is likely to be charged with manslaughter after admitting in a statement made public by his lawyer that one of the children he had with his own daughter died shortly after birth without medical attention.

Mr Fritzl, 73, is in custody after admitting to having kept daughter Elisabeth, 42, as a sex slave in a concrete bunker beneath his house for 24 years and to producing seven children with her.

A court in St Pölten, the capital of Lower Austria, today decided to extend his period on remand for a month after a closed hearing lasting less than 15 minutes. Gerhard Sedlacek, spokesman for the St Pölten prosecutors’ office, said that Mr Fritzl had waived his right to appeal against the extension of his custody.

“The investigation continues and Mr Fritzl will soon be questioned by the prosecutor. The charges against him will probably be filed this autumn, when we expect the trial to begin,” Mr Sedlacek told The Times.

After his remand was extended, Mr Fritzl reportedly requested a visit from his wife. He apparently inquired whether it was possible to receive visits and said he would like to see his wife, whom he “loved".

His wife Rosemarie, 68, is currently receiving counselling with her daughter Elisabeth and her seven grandchildren in a psychiatric clinic near Amstetten.

Prosecutors say that it is increasingly likely that Mr Fritzl will face charges of manslaughter or "murder out of negligence" for failing to provide medical attention for the newborn that died in the cellar where he imprisoned his family.

“We have read the interview and will subject Mr Fritzl thorough questioning. If it emerges that he was aware that his child was severely ill but still did nothing to bring help, that would be a case of murder out of negligence,” Gerhard Sedlacek, a spokesman for the St Pölten prosecutors’ office, said.

The baby, called Michael, was born with his twin on April 28, 1996, but reportedly died three days after that. Elisabeth confirmed that the baby had died shortly after birth.

Officers investigating Mr Fritzl’s three-storey house and the surrounding property announced that they will employ sniffer-dogs to inspect the garden and determine whether there were any corpses buried in it.

Archaeologists and other experts were also summoned at the scene to use a sonar probe and check for graves or more underground bunkers.

Colonel Franz Polzer, a police spokesman, said that investigators found two previously unknown rooms in the cellar next to the dungeon where the captive family were kept, which were not used for many years and sealed off with concrete.

Mr Polzer said that both rooms were filled with building rubble and could have served as a storage space during the construction of the dungeon. Experts have not yet inspected the rooms to determine whether they contained any evidence relevant to the investigation.

Cristoph Herbst, the lawyer hired by the state of Austria to represent the Fritzl family, announced that he would demand that the authorities to freeze Mr Fritzl’s assets in order to make them available for damages payments for his clients.

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