Berlin - The third day in the trial of Josef Fritzl, the Austrian father in court for imprisoning and raping his daughter Elisabeth for 24 years in the cellar of his home, held a number of surprises on Wednesday.

As the jury gathered at the start of the day, expecting to hear the psychologist's expert statement, Fritzl stepped into the witness box where he gave a full confession.

"I admit my guilt," Fritzl said in a barely audible whisper. Asked by the judge which charge he was referring to, Fritzl answered, "on all points."

The 73-year-old, who had already admitted to incest, false imprisonment and rape, had previously denied the charges of slavery, and the murder through negligence of one of Elisabeth's infant sons, who died within days of birth as a result of respiratory problems.

Fritzl said his change of mind, which even caught his defence lawyer by surprise, was the result of his daughter Elisabeth's video testimony, screened in the court Tuesday.

"I heard her statement yesterday, and I realized that I had made a mistake," he told the courtroom.

Another surprising fact to emerge Wednesday is that Elisabeth herself may have been present at Tuesday's hearing.

The speculation, published by Austrian daily Kurier, was not confirmed or denied by the court spokesman on Wednesday.

Fritzl's lawyer Rudolf Mayer said, "If she was in the courtroom, that could help explain his change of heart," referring to Fritzl's unexpected about-turn.

Elisabeth's son Michael, one of the twins fathered by Fritzl in the windowless dungeon, survived just 66 hours after Elisabeth's pleas for medical aid went unanswered.

Fritzl said he had not wilfully allowed the baby to die, but had misjudged the health of the infant. "I was of the opinion that the little one would survive," Fritzl told the court.

The Austrian also admitted that the conditions under which he held his daughter, in a cramped windowless dungeon below his home, amounted to slavery.

In late August 1984, Fritzl lured the 18-year-old Elizabeth into the cellar and in the years that followed fathered seven children with her, of whom six survived.

After telling the police his daughter had disappeared but didn't want to be found, Fritzl raised three of the six children above ground with his wife, claiming they had been left on the doorstep.

When Kerstin, one of the three children living below ground, fell critically ill in 2008, Fritzl took her and Elisabeth to hospital, where the authorities became suspicious.

The forensic psychologist took to the stand Wednesday, concluding the trial's hearings with her judgement that Fritzl's mental state allowed him to be held fully accountable for his deeds.

"Fritzl suffers none of the mental illnesses that would rule out his accountability," expert witness Adelheid Kastner said.

Fritzl had developed a "dangerous need for power and control" during his childhood, the forensic psychologist testified.

Her report found that the former engineer led a split life between the cellar where he kept his daughter and the rest of the world.

Kastner said, throughout the 24 years of his daughter's imprisonment, Fritzl had always acted "in the knowledge that it's not allowed to be," and that he knew his behaviour made him culpable.

But if he did ever experience guilt, Fritzl said during the trial, "Whenever I closed the door downstairs, it was gone." He only ever felt bad as he was going to sleep or waking up in the morning.

"There are two sides to Mr Fritzl," Kastner said, adding, "Both sides are in conflict."

The psychologist recommended that, due to his propensity for violence, Fritzl should be admitted indefinitely to a facility for abnormal offenders at the end of his sentence.

She said it was not foreseeable what period of psychiatric treatment would be necessary for him to no longer present a danger to his family.

The prosecution and defence are due to give their closing addresses to the jury Thursday, before the 12 jury members withdraw to consult on their verdict.

The court spokesman, Franz Cutka, said Wednesday that, under Austrian law, a judgement couldn't be passed purely on the basis of a confession from the defendant.

The evidence needed to be weighed up, he said, adding that Fritzl's admission of guilt could lead to a diminished sentence.

The charge of murder carries a lifelong prison sentence, while slavery - a charge pressed for the first time in Austria - carries 20 years.

A final verdict on the trial is expected Thursday.