A man who fathered nine children with his two daughters had attracted official suspicion but avoided detection for decades by moving his family frequently and intimidating his victims into silence, British prosecutors say.

Authorities were investigating on Wednesday exactly how the rapist was never called to account by neighbors, teachers, doctors, social workers, police or his extended family. The 56-year-old man was only apprehended when his daughters finally broke their silence.

He was sentenced Tuesday to life in prison for what a judge said was "the worst (case) I have come across" in 40 years of judicial practice.

Prosecutors told the court that the man raped his daughters from the time they were 8 or 10 years old, beating them when they resisted. Over a 25-year period he impregnated them 19 times, and they bore seven surviving children. Two more infants died at birth, while the other pregnancies ended in abortion or miscarriage. None of the family can be named because of reporting restrictions to protect the identity of the victims and their children.

Prime Minister Gordon Brown said Wednesday that people "will rightly want to know how such abuse could go on for so long without the authorities and the wider public services discovering it and taking action,"

Britain's child welfare services are already under pressure after the case of "Baby P," a toddler who suffered months of abuse that led to his death, despite repeated visits by social services staff in London.

"If there is a change to be made in the system and the system has failed, we will change the system as a result of the inquiries," Brown said.

Authorities in Sheffield, the northern English city where the family lived for a time, said they had begun an investigation.

"Where were the medical professionals? Where were the social workers? What were they doing for the last 20 years?" said Nick Clegg, leader of the opposition Liberal Democrats, who represents a district of Sheffield.

One answer is that some people did notice. At various points police, schools and medical staff all asked questions about the daughters' situation — but the daughters denied anything was wrong and the pieces were never put together.

The Sheffield case has echoes of the allegations against Josef Fritzl, the Austrian accused of keeping his daughter locked in a cellar for 24 years, fathering seven children by her. He is awaiting trial on charges of murder, rape, incest, false imprisonment and enslavement.

Unlike Fritzl, the British man moved his family frequently around northern and central England and lived in small villages to avoid drawing the attention of outsiders.

Prosecutors described how the defendant used intimidation, fear and evasiveness to keep his secret. He warned his children to keep quiet and when they were older he beat them. Each daughter said she was unaware the other was being abused until the pregnancies began.

"The defendant also ensured that his family were kept isolated and that there were very few visitors to the home," prosecutor Nicholas Campbell told Sheffield Crown Court.

"The victims were too frightened to tell anyone, even their mother," Campbell said. The court was told that the girls' mother left their father in the early 1990s, more than a decade after the abuse began, and it is unclear whether she knew what was going on.

The court heard that authorities at several points raised concerns about the family. A school asked questions about burn marks on one girl's arm, but it was attributed to bullying.

In 1997, the daughters' brother went to police to report the incest. But his sisters refused to cooperate and the investigation stalled.

Medical staff also had concerns about the high number of abnormalities in the women's pregnancies. One doctor even asked one of the women whether her father was the father of her children. She denied it.

But in June the women finally reported their abuse to social workers. Their father was arrested and last month pleaded guilty to 25 counts of rape. Judge Alan Goldsack sentenced him to 25 life sentences, to run concurrently, with no possibility of parole for almost 20 years.