A tenth of two to five-year-olds have a serious psychiatric illness, yet most cases are being missed, warn experts.
Early intervention is the way forward, experts believe
The problems go beyond tantrums and bad behaviour and impact negatively on all aspects of an infant's life, the Institute of Psychiatry will hear.
And failure to spot and treat these conditions early is causing unnecessary distress and suffering.
Mental health services need to be geared towards very young children as a matter of urgency, they said.
The news comes as a survey of 1,000 young people aged 12-19 by The Priory Group finds as many as one in five teenagers has considered or actually harmed themselves purposefully because of feelings of failure and social inadequacy.
Work by Professor Adrian Angold from Duke Medical Centre, currently in press, suggests most mental health problems are evident even in infancy.
He found that one in 10 children aged two to five, from a sample of 307, had obvious signs and symptoms of psychiatric illnesses such as attention deficit hyperactivity disorder (ADHD), anxiety or depression.
This is similar to the rate seen in older children, which he said was surprising.
It suggests that such conditions begin very early in life, perhaps even in the womb, Professor Angold, associate professor of psychiatry and behavioural sciences at Duke, told a London conference.
He believes that screening for and treating these disorders in babies and infants is the way forward - waiting until childhood or adulthood is too late.
For example, if someone has depression it can harder to treat if they have had it for a long time with lots of relapses, he said.
"What are our results are indicating is that this may mean intervening much earlier than people have typically thought.
"If you can start early with interventions then the secondary effects of the disorder itself - on schooling for example - may be ameliorated.
"We do not know how many of those kids are going to go on and continue to have problems. But I suspect they will."
He said paediatric psychiatric services should make more provision for treating pre-school children.
He suggested that nurseries and primary schools, as well as parents and GPs, should be on the look out for early warning signs of mental health problems in youngsters.
Professor Mark Dadds, professor of psychology at the University of New South Wales, Australia, said experience showed that for ADHD, early intervention could help.
"The problem is that access to treatments is poor."
Meanwhile, Professor Peter Jones, professor of psychiatry at the University of Cambridge, said that there was good evidence to suggest that factors in early life were important in determining a person's risk of developing schizophrenia and that intervening early, before adolescence, might help.
In particular, early cannabis use is associated with a two- to three-fold increased risk of schizophrenia, he said.
Professor Jones said the risk might be linked to duration of use or that there might be a critical window for harm during childhood development.
Alternatively, there may be other differences between children who use cannabis and who do not use this drug that explain the trend, he said.
The Priory Group survey found nearly half of Britain's teenagers had been offered illegal drugs like marijuana and a third had gone on to try them.
Marjorie Wallace, chief executive of the mental health charity SANE, said: "We are alarmed by the numbers being triggered into a drug induced psychotic breakdown by the availability and society's tolerance of street drugs, particularly chemical hybrids like skunk, and alcohol.
"For those who are genetically or otherwise vulnerable, being pushed to flashpoint at an early age can lead to life long mental illness."