Child refugees are still being locked up by the immigration service which wrongly classifies them as adults, a new report claims.
Young people can be left seriously damaged by their detention with some suffering mental health difficulties as a consequence, it warns.
Some highly vulnerable children who arrive in the country unaccompanied are held for weeks before officials accept they are not adults, it found.
The report, published this week by the Refugee Council, raises concerns about the number of age dispute cases more than two years after the Government vowed to end the detention of children for immigration purposes.
The charity believes officials are failing to exercise sufficient caution on the issue, meaning the controversial practice has continued.
It has called on the government to implement a number of safeguards to reflect the serious nature of a decision to treat someone as adult based purely on their appearance.
Donna Covey, chief executive at the Refugee Council, said: "It is a scandal that two years after the Government agreed to end the detention of children because of its harmful effects, they still believe it is acceptable to lock up children who have come here on their own.
"These are children who have fled horrifying situations in their own countries and have made traumatic journeys to reach safety here.
"They are then met with disbelief by the people who are supposed to help them and locked up with other adults in detention centres.
"The UK would never treat a British child in this way. We have an obligation to protect these children, so it is imperative that they are not held in detention and that they are given the benefit of the doubt."
According to the report, many refugees arrive in Britain on false documents - something which is accepted under international law. These are likely to be marked with an adult's date of birth.
While some children are advised to state they are older than their years to protect themselves from exploitation on their journeys, others would not be allowed to travel alone if their true age was known.
This means immigration staff are not immediately equipped with the necessary information and young people may be incorrectly judged to be an adult based simply on their physical appearance.
"One of the biggest issues young people seeking asylum face on arrival in the UK is that they are usually unable to verify their date of birth with official documents," the report states.
In the first three months of this year, six children were released from detention after officials realised they were under the age of 18, it found. Another four cases remain outstanding.
Last year, 22 children were released from detention for the same reason. This figure stood at 26 in 2010.
The charity's report, Not a Minor Offence, focuses on the cases of a number of children it claims were wrongly locked up.
Kabir, who fled Iran after his father was killed and his brother imprisoned for opposing the political regime, arrived in the UK aged 15.
Assessed to be an adult, he was kept in a police cell for four days before being transferred to an Immigration Removal Centre.
He was held for a month-and-a-half - unable to communicate - before his release was secured.
He still suffers from mental health difficulties because of his detention, according to the charity.
"The psychiatrist who assessed him stated that the six weeks he was locked up, when he felt abandoned and terrified about his future, were the cause of his current disorder, particularly because he was not able to mix with anyone of his own age," the report found.
Another child, Faisal, was 15 when he arrived in the UK from Afghanistan, having hidden inside a lorry. He was detained for more than three weeks following a journey which lasted several months.
The Refugee Council has outlined a series of measures to address the "pernicious situation of these forgotten children".
This includes asking the UK Border Agency to refer all asylum applicants claiming to be children for a local authority assessment and ensuring staff only decide someone is an adult if there is no possible chance the person in question could be a child.
If there is any doubt, the decision should be made not to detain, even if someone is treated as adult, the report suggests.
The Refugee Council works to secure the release of children from immigration detention when they are treated as adults.