NHS spending on children's mental health services in England has fallen by more than 6% in real terms since 2010, according to official figures.
The cut, equivalent to nearly £50m, was revealed by NHS England in a parliamentary answer.
Labour, which had asked for the figures, accused the government of breaking its promise to make mental health a priority.
Ministers point to an extra £7m spent this year on more psychiatric beds.
In addition, they say new investment is also planned to prevent eating disorders and self-harm.
Child and adolescent mental health services are under increasing pressure, juggling tight resources and with increasing demand.
In November, a 16-year-old girl from Devon spent two nights in a police cell as no psychiatric bed was available.
Investigations by BBC News and the online journal Community Care last year revealed an increasing number of children were being treated on adult wards while others were travelling hundreds of miles to get care.
Care Minister Norman Lamb has previously described children's mental health services in England as "not fit for purpose".
According to a parliamentary answer this week, NHS spending on children's mental health services in 2009-10, the final year of the Labour government, was equivalent to £766m (at 2013-14 prices).
In 2012-13, the last year for which figures are available, it had fallen to the equivalent of £717m.
The cuts by the NHS in England come on top of reduced spending by councils.
An investigation by the charity Young Minds last year found more than half of councils in England had cut or frozen budgets for child and adolescent mental health between 2010-11 and 2014-15.
Commenting on the funding cuts by the NHS, the chief executive of Young Minds, Sarah Brennan said: "These are deeply worrying figures.
"Children and young people's mental health services have been chronically underfunded for decades and the current cuts to their funding have just added to the crisis that many local services face.
"These figures along with YoungMinds' previous research demonstrates the 'double whammy' that children and adolescent mental health services face as both local NHS services and local government cut funding."
Luciana Berger MP, the shadow public health minister said: "These figures prove the government has broken its promise on mental health.
"Instead of making it a priority, ministers have cut the spending on children's mental health services each year since they came to power.
"The impact of these disastrous decisions has been devastating with increased waiting times, children having to travel hundreds of miles to get the help they need or being detained in police cells because there isn't a bed available for them.
"The government must take urgent action now to get to grips with this crisis."
Ministers say that overall NHS spending has been protected by the government and point to a five-year investment to prevent self-harm and eating disorders.
Mr Lamb said: "We have legislated for mental health to get its fair share of local funding but too often children's mental health still loses out.
"That's why I have brought together experts from across health, education and social care to look at how we can give children the best possible mental health care."
He said the government was also investing £150m in services for young people with self-harming and eating disorders.