Mental or behavioural problems which could blight the rest of their lives are suffered by up to one in five of the world's children, two United Nations agencies warned yesterday.
The World Health Organisation and the United Nations Children's Fund (UNICEF) said that rates of depression, suicide and self-harm had increased alarmingly among the young.
Children growing up in war zones faced the toughest problems, but rapid social and economic change in both developed and developing countries left other young people at risk.
Hans Troedsson, WHO director for child and adolescent health, said: "The international health community is concerned about the mental health status of our young. It is a time bomb that is ticking and without the right action now millions of our children growing up will feel the effects."
The agencies said that mental illness among children and teenagers, in particular, had for too long been a neglected area of public health. But in a joint report, they warned that children with depression were much more prone to other illnesses and risky habits which could shorten their lives.
Around 70 per cent of premature adult deaths were linked to behaviour that developed in adolescence, such as smoking, drinking and drug abuse, the report said.
Each year, nearly 1.5 million adolescents died from substance abuse, reproductive ill-health, suicide, injuries and violence. Adolescents also accounted for half of all new HIV infections, the report said.
The warning was endorsed by the Mental Health Foundation (MHF), a leading British charity, which will today call for every secondary school to develop a strategy on mental health. In a recent analysis, the charity also concluded that 20 per cent of children suffer some form of mental problem, including anxiety disorders and disruptive behaviour.
Boys are more at risk than girls, with a 64 per cent increase in the suicide rate among young men, aged 15 to 24, in the 10 years up to 1994. According to the latest government figures, 10 per cent of boys and six per cent of girls aged five to 10 have a mental disorder. The MHF will call for schools to give mental health the same priority as anti-bullying strategies.
According to the WHO, depressive disorders are the fourth leading cause of ill-health and disability among adults worldwide. But by 2020, mental disorders will represent the world's second biggest health problem.
In yesterday's report, the WHO and UNICEF also highlighted the lack of progress in reducing the high death rates among infants worldwide, many of whom died from preventable illnesses.