MI5 agents and anti-terror police have been given the go-ahead to question teenagers placed under the control of family court judges as a result of radicalisation fears.
Senior officers thought a judge might have to give permission before a child who had been made a "ward of court" could be interviewed.
But the most senior family court judge in England and Wales says investigators do not need to approach a judge before beginning questioning.
Sir James Munby has announced his decision in a ruling after analysing the issue at a hearing in the Family Division of the High Court in London.
A number of teenagers have found themselves at the centre of family court litigation, and been made wards of court, in recent years as a result of police and social services staff raising concerns about them becoming radicalised and travelling to Syria.
Judges can control youngsters' movements and decide whether they should have passports by assuming parental responsibility and making them wards of courts.
Sir James, President of the Family Division of the High Court, has concluded that police and MI5 agents do not need "judicial consent" before interviewing a child who is a ward of court.
He said council social services bosses had asked him to rule on the issue after an MI5 agent approached a teenager in their care who had been made a ward of court.
The judge did not name the council or the teenager in his ruling.