Children in care are too often missing out on treatment for mental health problems despite being four times more likely to experience them, say MPs.
The Commons Education Committee says children fostered in England are sometimes denied treatment simply because they move placement too often.
They should instead be given priority for mental health support, the committee says in a report.
The government says it is investing £1.4bn in children's mental health.
Almost half of children in care have a diagnosable mental health disorder, the MPs heard, compared with about one in 10 children who are not in care.
But provision for children in care with mental health problems is poor in many parts of England, says the report.
A significant number of local authorities are failing to identify mental health issues when children enter care, it adds.
And in some areas children in care can be turned away because their conditions are not deemed severe enough to qualify for treatment.
The MPs heard some child and adolescent mental health services (Camhs) were unwilling to begin treatment if a child moved into a new foster placement, even if this was within the same local authority.
"Given children in care may have unstable family lives and are frequently moving foster or residential placement, this inflexibility puts vulnerable children in care at serious disadvantage in getting the support they deserve," said Neil Carmichael, who chairs the committee.
"This must change - 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 one 16-year-old girl in foster care who had waited for more than two and a half years to be seen by her local Camhs.
In her evidence, given last December, the girl told the committee she had not been seen because she had moved placement 13 times.
Her foster carer told the committee the 16-year-old had been with her for 10 months and was still waiting for an appointment.
This was in spite of official guidance that states: "Looked-after children should never be refused a service, including for mental health, on the grounds of their placement."
Annie (not her real name), from Faversham, Kent, had been taken into care because she had not felt safe at home - but the experience was "traumatic" from the outset, she told MPs.
She said: "They took me out and they could not find me a placement at the start, so they expected me to go back to my family after they had taken me away, an hour later I said, 'I can't do that, I'm sorry. My life is at risk if I do that.'"
She had had 13 different placements in her first two and a half years in care, she said.
"I have locked everybody away," she said. "I went to so many [placements], I gave up giving myself to people."
Despite her need for counselling, the local Camhs would not give her an appointment because of her frequent moves.
"They will not move around with the child, which they should do," Christine Malone, Annie's foster carer for the past 10 months, said.
Annie missed six months of school and most of her GCSEs because of a placement on completely the opposite side of the county.
At school, she found it difficult to talk to teachers because of too much "baggage" and severe bullying.
She is now in college but told MPs: "It is like your work, college - you do not tell them everything about your personal life do you?
"It can be awkward as well, like with depression or self-harm, you are embarrassed and do not want people to know because you feel like they will judge you in some way.
"So you need Camhs to be there as well, or have one person and not loads of different carers to help you through school. I had Christine, lucky enough."
Similar incidents of Camhs refusing to see children without stable placements were described in several other pieces of evidence to the committee.
Too often, schools are left to try to help children with mental health problems with little support from other agencies, its report says..
It also calls for more mental health support for young people leaving care, who are more than five times more likely than their peers to attempt suicide.
The report urges:
- better mental health assessments of children entering care
- better mental health support for care leavers
- better integration of services for children in care, with more focus on mental health
- more mental health training for teachers, foster carers and children's home staff
- a bigger voice for children in care on the services they receive
- better research on the prevalence of mental illness in children
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 £1.4bn into children and young people's mental health, and investing in better links between these services and schools.
"This is backed up by £700m in reforming the social work profession, so staff are supported to make the right decisions for those in their care."
The Local Government Association's community wellbeing spokeswoman, Izzi Seccombe, said mental health services for children needed urgent improvement.
She said it was wrong for any child to have to wait a long time for treatment, but for the level of support needed, there had to be a joint approach with every organisation involved in a young person's life, such as schools, carers and health services, as well as councils.
If the government's vision to improve mental health services was to be realised, it needed to be backed with proper funding, she added.
Young Minds chief executive Sarah Brennan said the charity had long advocated giving children in care priority for mental health treatment.
They could have traumatic experiences but the impact on their mental health was too often ignored, she said.
"Yet these are the young people who too often go on to have the most severe and enduring mental health conditions throughout their lives," said Ms Brennan.
"If they receive the right help early on, as all the evidence shows, much of this long-term distress for them, and their friends and families, could be avoided."