Too many young criminals are being locked up - even by the Government's own criteria, a charity warned today.
Guidelines for youth courts suggest that those under 14 should be given custodial sentences only as a "last resort".
The law states children aged 14 and under should not be locked up unless they have committed a grave offence or have committed a serious offence and are deemed to be a persistent offender.
But a study by children's charity Barnardo's found that more than a third of 12 to 14-year-olds who were locked up did not meet the conditions.
The scale of the problem means that in just one year 170 youngsters in England and Wales were wrongly put behind bars.
Barnardo's chief executive Martin Narey said: "Until 1998 it would have been illegal to send a child of this age to custody unless they had committed one of the so-called 'grave offences'.
"Now we do this, every year, to more than 400 children aged 12, 13 and 14.
"This is a tragedy for the young people themselves, it's a shocking waste of money and, in terms of reducing their offending and doing anything to protect victims, it is almost invariably ineffective.
"We are calling for stricter, clearer rules on sending children as young as 12 to custody so that practice can be brought into line with government policy."
Mr Narey's comments came as a committee of MPs warned of the inconsistent use of custodial sentences.
The Justice Committee said there was no "common understanding" of what was meant by "last resort" and warned of "regional variations" across England and Wales.
Some young offenders were being locked up when there were not enough resources to provide community punishments, the committee said.
It warned that new guidelines for youth courts could mean children under 14 being locked up for three minor offences even if they have never appeared in court.
Committee chairman Sir Alan Beith said: "We know that custody does not work to reduce reoffending, and that it does not have a deterrent effect on young people, because their crimes are usually opportunistic and impulsive, so it is vital that effective alternatives are available."
Barnardo's surveyed around half of all children who were put in young offender institutions last year.
More than a fifth were locked up for breaching an Asbo or similar order, half were victims of abuse and more than a third were living with an adult criminal.
Liberal Democrat justice spokesman David Howarth said: "The Government's obsession with looking tough has led to large numbers of children being put behind bars when they shouldn't be.
"Prison should be the absolute last resort for the most serious and serial child offenders, not a default position for breaches of community sentences or Asbos.
"Locking kids from very difficult backgrounds up is likely to turn them into serious criminals as adults.
"Ministers should focus instead on what works to stop reoffending - such as restorative and community justice."
Juliet Lyon, director of the Prison Reform Trust, said: "We know that sending children to prison is expensive and ineffective, with three-quarters reoffending within a year of release, yet we still persist in sending thousands of our most vulnerable young people into corrosive youth custody every year.
"We need sentences that work, not those that lead children into more trouble."Reuse content