Children are being trafficked into the UK from Africa and used for human sacrifices, a confidential report for the Metropolitan Police suggests.
Children are being beaten and even murdered after being labelled as witches by pastors, the report leaked to BBC Radio 4's Today programme said.
Police face a "wall of silence" in investigations because of fear and mistrust among the groups involved.
It follows the case of a girl who was accused of witchcraft by her guardians.
Three people, including the girl's aunt, were convicted of trying to "beat the devil out of" the un-named 10-year-old - originally from Angola.
The report was commissioned by the Met after the death of Victoria Climbie in February 2000 and because of concerns over so-called faith crimes.
For spells to be powerful it required a sacrifice of a male child unblemished by circumcision
Police report
The 10-month probe was also intended to be part of efforts to "open a dialogue" with Asian and African communities to prevent child abuse in the London boroughs of Hackney and Newham.
Information was gathered with the help of social workers, human rights lawyers and race relations experts from within these ethnic minority groups.
Today programme reporter Angus Stickler, who obtained the police report due to be published later this month, described it as "absolutely chilling".
"The most gruesome details come from the African communities," he said.
"This report talks of rituals, of witchcraft, being practised in churches in London. It is described as big business."
'Dead meat'
It said that people who are desperate seek out churches to cast spells for them.
"Members of the workshop said for spells to be powerful it required a sacrifice of a male child unblemished by circumcision," the report said.
Contributors said boys were being trafficked into the UK for this purpose, but did not give details because they said they feared they would be "dead meat" if they told any more.
They always seem to base their models on the fact that Africans are less civilised, less rational Dr William Les Henry
There were also claims that youngsters were being smuggled into the UK as domestic slaves and for men with HIV who believed if they had sex with a child they would be cleansed.
One HIV outreach worker who spoke to the BBC News website said a small minority of Africans who came to his sessions had begun to mention this as a possible solution to their problems.
The authors of the report point out that these claims are only allegations.
They also claim that children could be in "serious and possible life-threatening situations".
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