A judge says experience has taught him to be "very circumspect" about the reliability of police intelligence.
Judge Iain Hamilton has made his comments in a written ruling after being asked to make decisions about the futures of two boys aged 14 and 15 at a private family court hearing in Manchester.
The judge, who has also overseen cases in criminal courts, said Manchester City Council social workers had been given police intelligence indicating that two of the boys' older brothers were involved in "gang activity".
Social services bosses wanted the judge to rule the boys should be placed in local authority care.
They argued that the older brothers' alleged "criminal activity" put the boys at risk of "emotional and physical harm".
But the judge refused to grant the care orders council staff wanted.
He questioned the "quality" of evidence the council was relying on.
"The contention that (the older brothers) are involved in gang activity stems from police intelligence provided to the local authority," said the judge.
"The court has no way of assessing how reliable or otherwise the police intelligence is."
He added: "In view of my past experience of sitting as a crown court judge for 14 years and the time I spent as the designated family judge here in Manchester ... I have learnt to be very circumspect about the reliability of police intelligence without more to support it."
Judge Hamilton said no-one involved could be identified.