Answering a question in parliament, youth justice minister Jeremy Wright revealed that five young people under the age of 18 were “authorised to move” into the adult secure estate during 2011.
Figures for 2012 and details on the circumstances of the moves in 2011 were not provided.
However a Prison Service spokesman said the practice is usually only used if a young offender is behaving badly.
"In exceptional circumstances, any young person can be transferred to an adult prison before their eighteenth birthday,” the spokesman said.
“This will usually happen because a young person’s violent or disruptive behaviour presents an unacceptable risk to the welfare of other young people."
The revelation has sparked criticism from campaign group the Howard League for Penal Reform, which has accused government of breaching the United Nations Convention on the Rights of the Child (UNCRC).
Article 37 of the UNCRC states that “every child or young person who is locked up must be separated from adults, unless it is better for him or her to be with adults”.
Andrew Neilson, director of campaigns at the Howard League, described the practice as “a cause for great concern”.
“The reason we have a youth justice system is that children have different needs to adults and must therefore be treated differently,” he said. “An adult prison is unsuitable and unsafe for someone under the age of 18.
“Detaining children in adult prisons makes them more vulnerable and puts them at greater risk of abuse and exploitation.”
Penelope Gibbs, chair of the Standing Committee for Youth Justice, said there should not be any circumstances where under 18s are accommodated in adult prisons.
"Staff need to protect the welfare of all children in the secure estate but it is possible to achieve this while also treating and looking after children with violent behaviour," she said.
"The training of staff in the secure estate should focus on how they can prevent disruptive behaviour and help these troubled children change"