LONDON (AFP) - British Prime Minister Gordon Brown voiced anger Wednesday over how authorities failed to spot a harrowing trail of sexual abuse which has been likened to the case of the Austrian cellar rapist Josef Fritzl. Brown said the country was "utterly appalled" by the case of a 56-year-old man who raped his two daughters and fathered nine children but went undetected for a quarter of century until finally being brought to the attention of the authorities in June. The case has focused the spotlight on the failings of social services, days after another scandal over a baby who died following months of horrific abuse. Police and social services in South Yorkshire and Lincolnshire in northern England launched an investigation into the sexual abuse case after the man was jailed for life Tuesday for making his daughters pregnant 19 times in almost 25 years. The trail of sexual abuse has chilling echoes with the case of Fritzl, who imprisoned his daughter underground for 24 years and fathered her seven children. The judge, who sentenced the unemployed British man for life at Sheffield Crown Court, said it was the worst case he had dealt with in 40 years. The man, who cannot be named to protect his victims' identities, evaded detection by regularly relocating his family to isolated villages to prevent the girls remaining in one place long enough to raise suspicions or form friendships with outsiders. Brown promised any necessary changes would be made to systems designed to protect children from abuse, telling lawmakers: "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. "And 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." During the trial in Sheffield, it emerged the desperate sisters called an emergency telephone line designed to allow children to report abuse confidentially. But when they asked for a guarantee that they would be able to keep their children if they informed the authorities of their father's identity, they were refused and they ended the call. Prosecutor Nicholas Campbell said that when abnormalities in the sisters' pregnancies became apparent during screening, one admitted they were made pregnant by the same man but flatly denied to doctors that it was their father. And in 1997, police investigated the family after the sisters' brother, who left home aged 15, made claims of sexual abuse, but no action was taken. Nick Clegg, the leader of the Liberal Democrats, Britain's second-largest Opposition party, said the daughters' school should also be asked why it failed to alert the authorities. "The girls went to school. Did they not notice anything?" he said. The sexual abuse case came as local authority chiefs in a district of north London continued to fight off calls for their resignation over the systemic abuse of Baby P, who was tortured to death by his stepfather and mother. The jury at their trial this month heard that social workers, police and health professionals in Haringey Council failed to spot the abuse of the 17-month-old boy. The Sun said it had gathered one million signatures for a petition calling for the sacking of all the social workers involved in the case, as well as a paediatrician who failed to spot that the child had a broken back. The same council failed to stop the death of eight-year-old Victoria Climbie from the Ivory Coast whose case similarly shocked Britain in 2000.