Bridgend MP Madeleine Moon and the children's commissioner for Wales urged the assembly government to fund a community mental health service.
Inadequate mental health care provision at a young offenders' institution has been criticised as scandalous by Wales' children's commissioner and an MP.
Both Keith Towler and the MP Madeline Moon have demanded urgent action by the Welsh Assembly Government to address the issue at Parc Prison in Bridgend.
Currently, young offenders with mental health care needs have to be sent to England to serve their sentences.
The assembly government said it was committed to support young offenders.
Mr Towler and Ms Moon have called on the assembly government to set up a community mental health service at Parc, the only institute for young offenders in Wales.
The category B prison, Wales' only private jail, has more than 1,000 male adults and young offenders.
Primary health care, such as GP services, is provided to the private prison by Primecare, but other health services such as dealing with serious and enduring mental illnesses are the Welsh Assembly Government's responsibility.
Parc Prison has never had an in-reach mental health service... and we have a serious problem here
Madeleine Moon MP, Bridgend
"Parc Prison has never had an in-reach mental health service - a child and adolescent mental health service - reaching into the prison and we have a serious problem here," Ms Moon told the BBC's Politics Show Wales.
"Parc was opened specifically so that Welsh youngsters could serve their sentence in Wales as close to their families and communities.
"Those most needy youngsters with mental health problems have had to be sent to England," she added.
Ms Towler agreed with Ms Moon's view that the situation was a national disgrace.
"These are children in need regardless of their behaviour and they deserve a response that can meet that mental health requirement," said Mr Towler.
"The stark fact is that we do not have those resources available.
"It is a scandal," he added.
Levels of serious mental health illnesses are believed to be higher among young offenders and female prisoners, according to professionals working in mental health care.
Reports dating back to 2002 have outlined concern of a lack of adequate mental health care.
A recent report by the independent monitoring board reiterates the need for adequate mental health care at the prison.
It said: "It is of concern that young people who arrive at Parc with serious mental health problems are transferred to establishments in England - often further away from family support."
It continued: "The board requests the Welsh Assembly Government to fund a community adolescent mental health service as a matter of urgency."
In a statement, the assembly government said: "Where offenders need access to specialist mental healthcare services, they could be referred to Glanrhyd Hospital, Bridgend.
"It is for the health professionals in the prison to determine where it is most appropriate to send the offender given they need to consider security issues."
It added that the £4.5 m it spent each year on children in the criminal justice system showed its commitment to support those young offenders.
The Politics Show Wales is on BBC 1 Wales at 1200 GMT on Sunday.
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