Baby P's mother named her violent partner as her next of kin on an official form but UK authorities still failed to realise he was living with her, a previously secret report has revealed.
The toddler's social worker was told Tracey Connelly had a boyfriend but did not ask who he was or request to meet him, according to a damning serious case review into the tragedy.
Baby P - now named as Peter Connelly - was just 17 months old when he died in Tottenham, north London, at the hands of his mother, her abusive partner Steven Barker and his brother Jason Owen in August 2007.
He suffered more than 50 injuries despite receiving 60 visits from social workers, doctors and police over an eight-month period.
The two serious case reviews into Peter's death, published in full for the first time on Tuesday, disclose that Connelly named Barker as her next of kin on health records in mid-2007 and described him as a friend.
The second report, commissioned after the first was judged by Ofsted to be "inadequate", was scathing about social workers' failure to quiz Peter's mother about her boyfriend.
It said: "It would have been interesting to know whether Ms A (Connelly) would have refused the information and what it was thought to signify. She was not asked."
The serious case reviews were published in full following pledges by Conservative and Liberal Democrat politicians.
Children's Minister Tim Loughton said he wanted the release of the reports to bring "some form of closure" so everyone involved in the case could move on.
He added: "The government's commitment to publish full SCR (serious case review) overview reports has always been about transparency, so that vital information is made available, so that agencies can be held to account and lessons properly learned."
The first Baby P serious case review, published in November 2008, was chaired by Sharon Shoesmith, who was later sacked as director of children's services at Haringey Council in north London over failings exposed by Peter's death.
It found "numerous examples of good practice" in the toddler's care, although it also noted some weaknesses in information flow.
But Ofsted inspectors ruled this report was inadequate and former children's secretary Ed Balls commissioned a fresh investigation.
Downing Street described the report as "shocking". David Cameron's spokesman said the British Prime Minister had been briefed on it, adding: "It's clearly a shocking report."