The council responsible for the care of the girl shot dead with her father, a convicted crack dealer, broke child protection rules.
Birmingham City Council's social services department should have informed its colleagues in London that it was moving Toni-Ann Byfield to the capital in August, but failed to do so.
The seven-year-old was shot in the back at the bedsit of her father, Bertram Byfield, 41, at Harrow Road, Brent, north-west London, on Sunday in an apparently drug-related murder. Police believe the girl was killed to prevent her from identifying the gunman.
An inquiry has been launched into the care of Toni-Ann, who was under the legal protection of Birmingham City Council. The council, which obtained an interim care order to gain legal guardianship of the Jamaican girl, moved her from a foster home in Birmingham to London just weeks before the shooting.
She was supposed to be living with a women described as her "aunt", but they are not thought likely to have been related.
Under the Children Act, legislation brought in to protect vulnerably youngsters, Birmingham should have informed the north London borough of Brent that staff were placing an at-risk child in their neighbourhood. But a spokeswoman for Brent Council said that the first time the authority knew of the existence of Toni-Ann was when news of her murder was announced by the police.
David Behan, president of the Association of Directors of Social Services, said: "It is a requirement of Children Act regulations that when a local authority has a child under its care who moves into another area, the relevant local authority should be notified."
Birmingham received the lowest, "no star" rates in last year's assessment by the Social Services Inspectorate. It said: "Birmingham's children's services were still not serving people well.".
Toni-Ann's mother, Christine Richards, 32, who is living in Jamaica, said she sent her daughter with a friend, Marcia Ashley, to live in Britain in 1999.
She said she did not hear from them for a year, but that they were living with another woman, believed to be Toni-Ann's half-sister, in a flat in a run-down part of Birmingham.
In November last year the city council's social services department placed her with foster parents. She was moved to London in August and a social worker travelled to visit her on "at least one occasion", according to the city council.
Mrs Richards said: "I blame the social services ... It's not even one month yet since she left their care, and now, she's dead." Birmingham City Council yesterday refused to discuss the case.
A 23-year-old man arrested and questioned by police over the murders has been bailed until a date in October but remains in custody in connection with suspected immigration offences.