"More than a quarter of children referred to mental health services in England in 2015 - including some who had attempted suicide - received no help, a report says.
A review by the Children's Commissioner also found that 13% with life-threatening conditions were not allowed specialist support.
She said the system was "playing Russian roulette" with their health.
NHS England said it was "clearly the case" that services need to expand.
The commissioner obtained data from 48 of England's 60 child and adolescent mental health service trusts, and discovered 28% of child referrals were denied specialist treatment - mostly on the grounds that their illness was not serious enough."
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