The Ministry of Defence wastes up to £94 million a year training 16 and 17-year-old army recruits for roles which could be filled by adults, campaign groups have said.
According to a report by Child Soldiers International and ForcesWatch, it costs as much to train a 16-year-old as an 18-year-old.
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Armed police were out in force as runners competed in the largest public event in the city since Monday's terror attack.
Taliesin Myrddin Namkai Meche, 23, was stabbed along with Rick Best, 53, when they tried to stop a racist attack on two Muslim girls.
The shadow home secretary, 63, said she worked in the Home Office for nearly three years when she was a trainee.