Badly behaved toddlers are being branded mentally unwell, a major NHS report reveals which claims that one in 18 preschoolers is suffering from a psychological condition.

The first official survey of children’s mental health since 2004 concludes that hundreds of thousands of two to four-year-olds have “at least one mental disorder”.

The results were last night described by charities as painting a “harrowing” picture of juvenile suffering.

But the report was also criticised by experts for inappropriately “medicalising” unruly behaviour. Based on information from 9,117 children and young people, health chiefs say 5.5 per cent of preschool children now have a mental health disorder.

However, nearly half of these were characterised as “behavioural”, with the single most common form being “oppositional defiant disorder” (ODD).

The concept was devised decades ago in the US, but it was not fully adopted by England’s National Institute of Health and Care Excellence until 2013.

The watchdog recommends that parents of affected children attend NHS-funded parenting classes.