Health services and councils are often failing to provide support for children with mental health issues in social care, MPs have found.
A report by the Commons Education Committee found a "significant number" of child and adolescent mental health services (CAMHS) are turning away vulnerable young people for not meeting diagnostic thresholds or being without a stable placement - something experts told the inquiry was "disgusting and a huge self-esteem blow".
The report said current methods of assessing children and young people's mental health and wellbeing as they enter care are inconsistent and "too often fail to identify those in need of specialist care and support".
The findings, based on an inquiry launched in September 2015, also revealed initial assessments are rarely completed by qualified mental health professionals.
Committee chairman Neil Carmichael said: "Local authorities have a special responsibility for the welfare of looked-after children. In spite of this duty, it's clear that many looked-after children in England are not getting the mental health support they need.
"We recommend children in care be given priority access to mental health assessments and never refused care based on their placement or severity of their condition".
The committee heard evidence from a range of service users and experts involved in the social care system, including the National Children's Bureau, which said that initial health assessments on entering care are "highly variable and often poor".
A 16-year old girl in foster care said she had been unable to access services for two-and-a-half years because she moved 13 times during that period. The committee heard that CAMHS are often unwilling to begin treatment if a child moves placement, even when this is within the same local authority.
It follows a health committee report, published in November 2014, that concluded that "there are serious and deeply ingrained problems with the commissioning and provision" of CAMHS.
Today's report said: "Looked-after children who need access to mental health services often have numerous and complex issues that require specialist input across multiple agencies.
"We have heard evidence that CAMHS is often unable to provide this care due to high thresholds and a refusal to see children or young people without a stable placement. The inflexibility of CAMHS is failing looked-after children in too many areas and leaving vulnerable young people without support."
The report said CAMHS support should be extended to all young people in care up to the age of 25 - an increase from the current cut-off point of 18.
Malcolm Trobe, interim general secretary of the Association of School and College Leaders (ASCL), said: "We recognise that the government has pledged greater investment in mental health services. However, we are extremely concerned about a rising tide of mental health issues among young people.
"More action is urgently needed to ensure there is sufficient specialist provision in all areas of the country."
Izzi Seccombe, the Local Government Association's community wellbeing spokeswoman, said: "We recognise that improvements urgently need to be made to the mental health services available to all children, in particular those in care.
"Early intervention is key, and it is not right for any child to have to wait a long time for treatment.
"However, to provide the level of support required, we need a joint approach with every organisation involved in a young person's life, such as schools, carers and health services, as well as councils.
"Clearly more investment is needed if we are to deliver the mental health support our children and young people need and deserve."
A Government spokesman said: "Children in care have often lived through traumatic experiences and it is vital they receive the support they need.
"That's why we are putting a record £1.4 billion into children and young people's mental health, and investing in better links between these services and schools.
"This is backed up by £700 million in reforming the social work profession, so staff are supported to make the right decisions for those in their care."