Officers investigating the Josef Fritzl incest and sexual abuse case revealed yesterday that he started building the underground dungeon in which he imprisoned his daughter six years before her incarceration began .

Speaking to The Times at a press conference today, Colonel Franz Polzer said that Mr Fritzl had planned the imprisoning and sexual abuse of his Elisabeth in astonishing detail.

"Fritzl acted with premeditation when he began building the underground cellar of his home in 1978," he said.

"We assume he had already selected his daughter Elisabeth who was to become a prisoner of the concrete dungeon."

Fritzl, 73, is accused of imprisoning his daughter , now 42, in a purpose-built dungeon beneath his house, where he sexually abused her as she gave birth to seven of his children.

Col Polzer also revealed that investigators, who have so far been focusing on examining the scene of the crime, Fritzl's three-storey family house in the town of Amstetten, have found a second entrance to the dungeon.

The 500kg iron and concrete door leading to the second entrance to the dungeon was so well-concealed and difficult to open, that police had to use heavy machinery borrowed from the local fire fighting brigade.

"The door was hidden in a room beneath the stairs that could be approached from the garden. The room resembles a setting of a horror film, with countless pipes and electrical cables and the door, which weights about 500 kg and is only one metre high, is so difficult to detect, that it is reminiscent of something from the Harry Potter books," Col Polzer said.

Police, however, believe that the second entrance had not recently been used, as the door was so heavy that it had become impossible to open it during the last three decades.

They confirmed earlier reports that Elisabeth was locked up in a single room of only 35metres squared, with an improvised shower and toilet for nine years, from 1984 until the birth of her fourth child in 1993, when Fritzl expanded the dungeon with additional two rooms to create a space totalling 55msq. It suggests that her children would witness Fritzl's sexual assaults on their mother.

It was also revealed that anyone trying to access the dungeon would need to pass through a total of eight doors to get there; all of them were locked and the last two were electronically controlled. The second entrance, only discovered on the weekend, was secured by four doors.

"Fritzl planned every detail with astonishing precision. His diabolical plan was almost perfect," Col Polzer said.

Despite establishing that Fritzl's son Josef, 37, who lived with his parents in recent years, had the key to two of the doors in order to be able to access the boiler room, police denied that he could have known about the dungeon, as only his father held all eight keys and the codes necessary to open the last two.

"There is no reason to believe that this very energetic man, who had focused all of his energy on his unfathomable plan, would involve anyone in the realisation of his perfect crime," Col Polzer said.

At the press conference it also emerged that Elisabeth, and her children, aged 18, 15, 14 and 12, as well Rosemarie, 68, Fritzl's wife, were in good condition.

Dr Bernhard Kepplinger, the head of the hospital where the Fritzl family are treated, said: "The family are developing a good social interaction and their condition is good and improving. The mother and the grandmother are preparing the meals together; the children are getting into a daily routine of making their beds and are spending time together. The family gets together at breakfast and dinner.

"The skin of the mother and the three children that have not been exposed to sunlight is now assuming a normal colour. They are still oversensitive to sunlight but are improving.

Dr Kepplinger also said that, during their years in the cellar, the family was supplied with Vitamin D supplements and even a UV lamp by their father, who also installed an aquarium with goldfish in their dungeon.

The condition of the oldest daughter, who is in an artificially induced coma and suffering from a yet undiagnosed condition, is also said to have improved and is no longer life-threatening.

In a separate development, Austrian legal experts revealed that the children Fritzl fathered with his daughter could sue him for millions in damages, after it emerged that their therapy after the decade-long ordeal in the dungeon will cost over one million euros.