A nine-year-old child asylum seeker arrived to the UK unaccompanied and was housed in a hotel alone by Home Office officials, a High Court judge has been told.
Mr Justice Johnson heard a "high proportion" of asylum seekers initially accommodated in hotels are unaccompanied children under 16.
He was given Home Office figures while overseeing a preliminary hearing in a High Court case centred on the use of a hotel to house unaccompanied children, involving the Government department and Brighton and Hove City Council.
The council is challenging a Home Office plan to re-use a hotel in the Brighton area from which migrant children have previously gone missing.
Bosses have taken legal action against Home Secretary Suella Braverman and want a judge to rule that the hotel should not be used.
They say children could be targeted by criminals and abusers.
Mr Justice Johnson was told the Home Office had stopped using the hotel but was planning to use it again.
A trial is expected to take place later this summer.
Mr Justice Johnson heard Kent County Council and a children's rights organisation are planning similar legal action relating to the accommodation of unaccompanied migrant children in hotels.
He heard that both Brighton and Kent councils were concerned the placement of unaccompanied migrant children in hotels puts them in a position where they are breaching, or would breach, their legal obligations relating to the care of youngsters.
The judge indicated all three cases might be considered together.
He asked a barrister leading the Home Office legal team the age of the youngest child to be initially accommodated in a hotel.
Lisa Giovannetti KC told him the youngest had been nine.
Mr Justice Johnson asked: "You have put a nine-year-old child in a hotel?"
Ms Giovannetti indicated the child had been in a hotel for 24 hours before moving to more suitable accommodation.
The judge then asked how many "under 16-year-olds" have been accommodated in a hotel for more than 48 hours.
"Quite a high proportion are under 16," said Ms Giovannetti. "Maybe 40%."
Stephanie Harrison KC, representing Brighton council, told the judge: "Children who are under 16 who are not in accommodation that is regulated - it is not only unlawful, it is a criminal offence. That encompasses the gravity of the situation."
Kent council's barrister Hugh Southey KC told Mr Justice Johnson: "Some local authorities are trying to do all they can to comply with their obligations.
"They are attempting to accommodate more children than they are obliged to - Kent is one of them.
"Other local authorities are not. They are taking trivial numbers of children."
A Home Office spokesman said later: "Due to the rise in dangerous small boats crossings, the Government has had no alternative but to urgently use hotels to give unaccompanied asylum-seeking children arriving in the UK a roof over their heads.
"The wellbeing of children and minors in our care is an absolute priority. Robust safeguarding and welfare measures are in place at all temporary hotels to ensure children are safe, secure and supported as we urgently seek placements with a local authority."
