Children who have been refused asylum should no longer be detained while awaiting deportation, the children's commissioner for England has said.
Sir Al Aynsley-Green warned in a report that children found Yarl's Wood Immigration Removal Centre, in Bedfordshire, "like being in prison".
Each year 2,000 children are held at the centre for an average of 15 days.
The government said detention of children was a necessity if their parents were refusing to return home.
Sir Al warned that detention "frightened" children, who became anxious about losing belongings and friends.
While he acknowledged the practice would not end overnight, his report set out 42 recommendations for making the detention of youngsters a "last resort".
"This has to end", said Sir Al. "We are particularly concerned at what appear to be significant discrepancies between policy guidance and what happens in practice to children during detention.
"Ultimately, we want any child who comes into contact with the asylum and immigration services to be treated fairly and humanely."
Sir Al recommended that babies or children who have serious health problems are never confined.
He suggested developing a community-based alternative along with government monitoring of children.
The report has the backing of children's charity Barnardo's.
Its chief executive Martin Narey, the former head of the Prison Service, said: "Children who have committed no crime should not and do not need to be locked up.
"It may be administratively convenient for the Home Office to do so, but it is as unnecessary as it is shameful.
"Parliament has long expressed reservations for terror suspects to be held without charge, yet up to 2,000 children are detained in a prison-like environment for immigration purposes each year."
Amanda Shah, of Bail for Immigration Detainees, a charity which helps represent families in Yarl's Wood, said: "The trauma experienced by children in detention comes across very strongly in this report.
"They describe being transported in caged, urine-soaked vans, separated from parents and not being allowed to go to the toilet. There is no proper provision to deal with their psychological distress, directly caused by the government's detention policies."
But immigration minister Phil Woolas said: "We only detain those who refuse to comply with the decision of the courts and do not leave Britain voluntarily. The report overlooks this vital point.
"If people refuse to go home then detention becomes a necessity. We don't want to split up families, so we hold children with their parents, and while they are in our care we treat them with sensitivity and compassion.
"We now have full-time independent social workers, and a range of trained experts to monitor welfare 24 hours a day."
Sir Al said any fears that families would disappear if not locked up, needed to be proved.
"I would like to see the hard evidence from government to support that assertion.
"Put yourself in the place of a family with young children. Where will they run to in this country?"
He said more evidence was needed on why children were being locked away, as well as looking at alternatives.
"If they have to be locked away then let's make it a humane process which fulfils common standards of decency," he told BBC Radio Four's Today.