Fifteen 17-year-old British soldiers were inadvertently sent to Iraq despite a UN convention to keep children away from armed conflict, it has emerged.
The government said no 17-year-olds had been deployed since 2005
In a written answer, defence minister Adam Ingram said the deployments took place between June 2003 and July 2005.
He said most soldiers went just before being eligible at 18 or were taken off duties less than a week after arrival.
Lib Dem education spokeswoman Sarah Teather said the government had shown a "shocking level of incompetence".
Mr Ingram said the UK ratified the Optional Protocol to the Convention on the Rights of the Child on the involvement of children in armed conflict on 24 June 2003 to ensure that under-18s were not deployed to war zones.
"Unfortunately, these processes are not infallible and the pressures on units prior to deployment have meant that there have been a small number of instances where soldiers have been inadvertently deployed to Iraq before their 18(th) birthday," he said.
He said all those deployed were 17 years old, and as many as four were girls.
But none have been deployed since July 2005 and fewer than five 17-year-olds were there for greater than three weeks.
Ms Teather, who requested the information, called on the prime minister to make an apology.
She said: "This is an inexcusable blunder by the government that reveals a shocking level of incompetence.
"We have rules about sending those under 18 into conflict for a reason.
"There is no way people so young are mentally or emotionally prepared to face bloodshed on the scale seen in Iraq."