Adolescents with mental health problems in Devon have been sent to units in Hull, Newcastle and Lancashire amid an NHS "crisis".
Several vulnerable young people in the county have also been admitted to adult psychiatric units.
Ben Bradshaw, Labour MP for Exeter, who has revealed the concerns of health professionals in the region, has written to Health Secretary Jeremy Hunt urging him to tackle the "unacceptable" situation.
Rising demand and a shortage of residential places for those suffering from the most serious problems – such as violent behaviour or life-threatening eating disorder – has been compounded by a massive shake-up in the NHS, creating problems with out-of-hours care, he claims.
NHS bosses in Devon admit "on occasion" young people have been admitted to adult mental facilities, an issue they are addressing.
Mr Bradshaw, a former health minister in the Labour government, said in the letter seen by the Western Morning News: "Three problems seem to have come together to cause what one senior local mental health provider described to me as a 'crisis' in care for vulnerable young people."
Responsibility for child and adolescent mental health services has shifted since the Government abolished local primary care trusts in April.
New clinical commissioning groups in Devon, headed by GPs, now licence care for young people with severe mental health problems, such as schizophrenia, through professionals including psychiatrists.
Known as "tier three" care, this is provided by Richard Branson's Virgin Care, which has run core NHS and social care services for children and young people in Devon since earlier this year.
The major concern surrounds more serious cases, or "tier four" services, requiring admission to day units or overnight residence, which fall to national commissioning board NHS England. It also handles out-of-hours support.
In his letter, Mr Bradshaw partly blames a "lack of clarity about where responsibilities lie". He writes: "Nobody appears to be responsible for commissioning the out-of-hours 'assertive outreach' services that ensures a young 'tier three' patient who is in crisis or having an episode out-of-hours or at weekends is looked after effectively and found a 'tier four' bed."
A spokesman for NHS England and Northern, Eastern and Western Devon Clinical Commissioning Group said as the two commissioners involved with child and adolescent mental health services (CAMHS) they work closely to ensure "young people's needs are met appropriately".
"We are aware that there has been an increase in demand in the number of young people who require inpatient CAMHS and this, along with a reduction in the availability of beds, has caused delays to the service and on occasion meant individuals may have been temporarily admitted to a local adult inpatient ward while a CAMHS bed becomes available," he said.
"This is something that providers and commissioners in Devon are working with clinicians to address."
