A Met report saying children are being trafficked into the UK from Africa and used for human sacrifice has been dismissed as a "racist witch-hunt".
Lee Jasper said the report lacked facts
Children are beaten and murdered after being labelled as witches by pastors, the report leaked to the BBC said.
But senior mayoral adviser Lee Jasper described it as alarmist and urged the force to withdraw the report.
A Met statement said it had spoken to Mr Jasper about the damage caused by the leaking of the 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 boroughs of Hackney and Newham in east London.
Information was gathered with the help of social workers, human rights lawyers and race relations experts from within these ethnic minority groups.
Radio 4's Today programme reporter Angus Stickler, who obtained the police report due to be published later this month, described it as "absolutely chilling".
"This report talks of rituals, of witchcraft, being practised in churches in London," he said.
The report said: "Members of the workshop [that produced the report] said for spells to be powerful it required a sacrifice of a male child unblemished by circumcision."
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.
But Mr Jasper told BBC News: "It [the report] lacks any kind of rigour in its findings.
"We think the conclusions are alarmist. They are not based on fact and the Met has to withdraw or rebut what is a very dangerous report.
"What is happening is tantamount to a racist witch-hunt of African communities in London."
A statement from Met said: "The commissioner has spoken to Lee Jasper and made it clear that this report was passed to a journalist without the authority or approval or consideration of the Metropolitan Police service.
"We are very aware of the damage that has been caused by linking this report with other cases and research and share Lee Jasper's concerns about its impact on communities across London.
"There is no evidence either in this report or elsewhere to support the suggestion that 300 african boys are missing in london.
"We intend to consult with our independent advisory group, African communities and the bishops in London to fully understand the perception of those who took part in this small research project."