The spectral courtroom presence of Elisabeth Fritzl may have tipped her father into abandoning his defence case and admitting yesterday to the murder of their baby.

The question intriguing Austria yesterday was whether Ms Fritzl – who made a virtual appearance in Court 119 on Tuesday to testify against Josef Fritzl on a giant television screen – also entered the room in disguise to observe her 73-year-old father. Sources in the clinic where she is staying and other trial observers appeared to confirm: she was there in person.

The court spokesman Franz Cutka said: “I can neither confirm nor deny her presence during the in-camera sessions on Monday and Tuesday.” His memory about yesterday’s proceedings was clearer: he was certain Ms Fritzl had not been in court when her father pleaded guilty to all charges.

The return of Ms Fritzl could explain Fritzl’s astonishing change of heart. Having fiercely contested many of his daughter’s claims and pleaded not guilty to murder and enslavement on Monday, he is suddenly confessing to everything.

“I regret it,” the retired building engineer told Judge Andrea Humer. “Only yesterday did I realise how cruel I was to Elisabeth.” His daughter, now 42, was locked up for 24 years in an improvised dungeon in the family home, where she was raped up to 3,000 times and bore seven children – one of whom died soon after birth.

Fritzl’s words yesterday were as close to remorse as he has come. Only two days ago he was choking back tears in his opening courtroom testimony – but they were tears of self-pity as he spoke of his difficult childhood and his unloving mother.

Seeing Elisabeth, he told the judge, had made him think again. After watching 11 hours of her evidence on television, he was shattered and had sought out his assigned therapist, Pattrick Frottier, for counselling. It was then, he said, that he had decided to make a clean breast of matters.

But could he have sensed Ms Fritzl’s physical presence in the room? As he watched her recorded testimonial and listened to her voice floating across the wood-panelled courtroom, he was facing away from the public gallery where two or three people were sitting. “If Elisabeth were in the courtroom, then I think that would have really shaken him up,” Rudolf Mayer, his defence lawyer, said.

Despite the fuzziness of official statements, the balance of probability is that Ms Fritzl really was present. She is writing a book as part of her therapy, aided by notes scribbled during her incarceration on the back of Fritzl’s shopping bills. But if she was in court, it was not as a clandestine reporter, but as a victim. “Victims have the right to come to court and observe a trial,” Mr Cutka said.

Confessing to the murder of his son Michael, who died soon after he was born in 1996, means that Fritzl faces a possible life sentence – and a minimum of ten years if he is found guilty.

When he appeared in court yesterday he was no longer hiding his face behind a folder: he had prepared for his performance by blue-rinsing his hair and his demeanour was a combination of vanity – stroking dandruff from his jacket – and stage fright.

When Judge Humer asked whether he had anything to say before proceedings began, he strode towards the desk in the centre of the court and blurted out his changed plea. Mr Mayer, seemed as taken aback as everyone.

The verdicts are expected today.

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