Austrian prosecutors are debating whether to charge incest offender Josef Fritzl with slavery, in a last ditch attempt to maximise the time he serves in jail, according to press reports.

Mr Fritzl, 73 a retired engineer, has admitted to imprisoning his daughter Elisabeth for 24 years in a purpose-build concrete dungeon beneath his home in the town of Amstetten, where she was forced to give birth to seven of his children. Three of the children were selected to live with him and his wife Rosemarie, 68, in their upstairs apartment, while the other three and their mother were never allowed to leave the damp concrete bunker until they were rescued by police on April 26. One of the children, a baby boy called Michael, died shortly after birth and Mr Fritzl burned his body in an incinerator.

But despite his partial confession, it was revealed last month that Mr Fritzl could only be facing ten years in prison because Austrian law, unlike British law, does not allow for multiple convictions, and because the only charge certain to be proved in court was that of incarceration.

The prosecutors’ case was additionally weakened after it emerged that the children who spent their entire lives in the cellar prison could refuse to testify against their father.

Desperate to make sure that one the highest profile criminal cases in their country's legal history ends with an appropriate prison sentence, Austrian prosecutors are now considering whether they can charge Mr Fritzl with slavery, under Paragraph 104 of the Austrian Penal Code - a crime originally deriving from the country’s 19th century law, that carries a maximum sentence of 20 years in prison.

A number of prominent legal experts are said to have been tasked by prosecutors in utmost discretion to try to find a solution that would ensure a lengthier prison sentence for Mr Fritzl. It was a distinguished university professor of criminal law, who has not been named, who suggested the slavery charges, according to press reports.

Judge Kurt Leitzenberger, head of the St Pölten Regional Court where Mr Fritzl will stand trial, told The Times: “The case is singular in the country’s history and there is no legal precedent to draw experience from. Compiling the charges against the suspect is a delicate procedure for a number of reasons and prosecutors are carefully examining all possibilities.”

Gerhard Sedlacek, the Austrian prosecutors’ spokesman, said: “The content of the charges is has not yet been determined. It is expected that the charges will be completed by the end of September and the date for the trial could be set sometime in December.”

In a separate development, experts hired by the court determined this week that there were no additional hidden rooms in the dungeon and that Mr Fritzl was fully capable of building and furnishing it himself and without the aid others.

Mr Fritzl is facing a string of charges including rape, incarceration, incest and coercion, as well as manslaughter for the baby that died. But prosecutors fear that the manslaughter changes could be “extremely difficult” to prove because so much time had elapsed since the death of the newborn in 1996. There is a similar problem with the rape chares, for which there is lack of forensic evidence.

The only serious crime that Mr Fritzl is certain to be convicted for is incarceration, which carries a maximum sentence of ten years in prison.

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