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The prime minister has said any necessary changes would be made to the system in the wake of the abuse by a father who raped his daughters.
His comments follow calls by MPs in Sheffield who have demanded to know how the father's actions were able to go undetected by official agencies.
The 56-year-old man was jailed for life on Tuesday after fathering nine children by the two women.
Gordon Brown said people were "outraged" by the "unspeakable" abuse.
An independent review is investigating the contact health professionals, police and social services had with the victims.
During Prime Minister's Questions at the House of Commons, Gordon Brown said MPs and the public were "outraged" by the abuse.
Meanwhile, Sheffield MPs Nick Clegg and David Blunkett have called for the inquiry to uncover any failings in the system.
The father, who admitted 25 rapes, was sentenced to a minimum term of 19-and-a-half years by judge Alan Goldsack QC, who said the case was the worst he had seen in 40 years.
Nick Clegg, Lib Dem leader and Sheffield Hallam MP, said the inquiry needed to establish the role of professionals and social and medical workers in the case, and whether there had been failings in the system.
He said: "I can't imagine a better definition of being evil than torturing your own children in this abusive way. Some of the details are just heart-rending.
"Of course the question is how on earth did something like this happen without someone noticing?"
Asked by BBC News "Where were the medical professionals? Where were the social workers?", Mr Clegg answered: "Yes, we don't know. Those are the big questions that need to be asked.
"The other questions are for people at school. The girls went to school. Did they not notice anything?
"The girls were in and out of hospital - did they not notice anything?
"That's why I spoke last night to the leader of Sheffield City Council, to the Chief Constable of South Yorkshire Police and this all needs to be looked at completely independently.
"That's the crucial thing. They've got someone from outside to look at it independently and make sure that we know if there have been any major failings in the system that those are brought to light."
Labour's Sheffield Brightside MP and former Home Secretary David Blunkett said the inquiry needed to focus on agencies that could have intervened but did not.
He said: "Those who at least made an effort to do something should not be the ones who are pinpointed - it is those who did not who should examine their conscience."
Sheffield Crown Court heard the abuse started when the women were pre-pubescent.
The father moved the family from village to village in rural locations, including Sheffield and Lincolnshire, to keep them isolated and to avoid detection.
The court heard that none of the doctors, nurses or social workers the victims saw looked into why they kept getting pregnant.
James Baird, representing the defendant, criticised social services in both Sheffield and Lincolnshire, where the family had lived.
He said: "What is clear from the case is that the complainants and the children have attended many, many hospital appointments, but none of the medical experts have sought to probe these circumstances, and accepted the complainants' case that the father of their children was not their father."
Jayne Ludlam, director of young people's services at Sheffield City Council, confirmed a Serious Case Review had been launched, to be led by former civil servant Professor Pat Cantrill.
Peter Duxby, director of children's services at Lincolnshire County Council and chairman of the Lincolnshire Safeguarding Children Board, said since the events, child protection systems had been improved.
"Significant improvements have been made to ensure the safety of children in Lincolnshire," he said.
"In line with Lord Laming's recommendations in 2003, there has been a wholesale reorganisation of children's services and the establishment of a Lincolnshire Safeguarding Children Board which provides effective management and ensures proper standards for the safety of children are maintained.
"We will be co-operating with Sheffield in the completion of an independent review."
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