Two campaign groups on Tuesday called for Britain to stop recruiting 16 and 17-year-olds to its armed forces, a practice that has been abandoned by most countries.
Child Soldiers International and ForcesWatch said Britain's Ministry of Defence (MoD) wasted up to £94 million ($143 million, 110 million euros) a year by recruiting minors, who it said were twice as expensive to train as 18-year-olds.
But the MoD said the report ignored the benefits that a military career offered young people, and said it had no plans to change its policy.
Child Soldiers International -- which campaigns primarily against the use of child soldiers in armed conflicts in Africa and Asia -- said fewer than 20 countries continue to recruit soldiers from the age of 16.
"The large majority of countries worldwide now recruit only adults aged 18 or above into their armed forces," the London-based charity said.
"The UK is the only member of the European Union and only permanent member of the UN Security Council still recruiting from age 16."
In a new report co-authored by the British campaign group ForcesWatch, the charity said under-18s were more likely to join the infantry, where the risk of fatality in Afghanistan has been five times that faced by the rest of the army.
Young troops were also more likely to suffer post-traumatic stress disorder, sexual harassment and bullying, it claimed, while money is wasted on training minors because they take longer to develop their skills and have a higher drop-out rate.
The report argued that while under-18s are banned in Britain from many activities considered harmful -- including drinking alcohol and smoking -- they are allowed to risk their lives in the military.
The Ministry of Defence denied that it was wasting tens of millions of pounds a year by recruiting minors.
"We do not recognise the figures suggested in this report, which ignores the benefits and opportunities that a military career offers young people," a ministry spokesman said.
"We continue to actively recruit across all age groups and as part of our duty of care to our recruits no young person under the age of 18 years may join our armed forces without the formal written consent of their parent or guardian.
"There are currently no plans to revisit the government's recruitment policy for under-18s, which is fully compliant with United Nations Conventions."