An immigration removal centre has wrongfully detained disabled children and transports families in metal cages, the prisons' inspectorate has found.
HM Inspectorate of Prisons said children were detained for too long and left distressed and scared at the Yarl's Wood centre in Bedfordshire.
But it said the centre had improved its conditions in several other areas.
The UK Border Agency said most children were held "for a week or less" and it was committed to fair treatment.
The report, following an inspection in February, said the average time children were held at the centre had risen from eight to 15 days in the last two years.
In a recent four-month period it found 83 children had been detained for over 28 days.
It said: "An immigration removal centre can never be a suitable place for children and we were dismayed to find cases of disabled children being detained and some children spending large amounts of time incarcerated."
The report also found there was inaccurate monitoring of how long children were being held for, inadequate education and after-school facilities, and some families had been transported to and from the centre in caged vans.
The report says: "Despite the efforts of centre staff, prolonged detention was having a detrimental effect on the welfare and behaviour of children, whose fear and distress was strongly reflected in our children's interviews."
It recommended: "Children should be detained only in exceptional circumstances and then only for the shortest time necessary. Length of cumulative detention should be clearly and accurately recorded."
However the inspectorate did find there had been improvements in conditions in other areas.
The report said security arrangements "were more proportionate" and "the centre was brighter and better decorated" than when it was last inspected.
There had also been a new welfare officer appointed, the centre was safe and "there was little evidence of self harm".
Chief Inspector of Prisons Anne Owers congratulated the centre for the improvements but voiced concern over the plight of children there.
She said: "Significant concerns remain, particularly the lack of activity for detainees, which is a failure that we have identified across the immigration detainee estate.
"Even more worrying was the plight of children detained for increasing periods of time and with insufficient provision to meet their needs."
The UK Border Agency defended its record on immigrant detentions and said it was soon to "adopt a legal duty to promote children's welfare".
A spokesman told the BBC: "We're determined to treat children within our immigration system with fairness and compassion, that's why the majority stay in detention for a week or less.
"Often children are only detained when their parents try to frustrate the removal process detention then becomes necessary to maintain a robust but fair asylum system.
"Those with no right to be in the UK must return home, and last year we removed a person every eight minutes."
The Refugee Council, who campaign for immigrants' rights, described the report as "scathing".
A spokeswoman told the BBC: "Yet again there is scathing criticism of the detention of children.
"How much more evidence does the government need of the harm detaining children causes before it puts a stop to it once and for all?"
The charity Bail for Immigration Detainees (Bid) said the consequences of being detained would have an impact on children that lasted long after they were released.
A spokeswoman told the BBC: "The impact on their well-being as evidenced by their fear and distress noted by inspectors is simply unacceptable, and the legacy of this experience will last well beyond their period of incarceration."
She added: "We call for an immediate end to the deplorable practice of detaining children, which we believe can never be justified."
It is not the first time child welfare has been criticised at the centre.
In July 2005 another HM Inspectorate of Prisons report found children were being "damaged" by their detention there.
At the time, Ms Owers said an autistic girl of five had been held at Yarl's Wood and not eaten properly for four days and that education at the centre was "inadequate" and "depressing".