The Government is being urged to investigate the under-reporting of the deaths of children and adolescents in mental health units.
Charity Inquest, which represents bereaved families, is demanding a statutory notification when such a death occurs.
Its research suggests that since 2010 at least nine youngsters died in England, while receiving in-patient psychiatric care, a figure the Department of Health (DoH) is unaware of.
In an interview with BBC Panorama Alistair Burt, Minister for Community and Social Care (England), admitted he did not know how many children and adolescents had died in the care of psychiatric units in recent years.
In a parliamentary answer last year Mr Burt suggested there had been only one such death recorded by the Care Quality Commission. His predecessor, Norman Lamb, said there had been none.
But Inquest uncovered the nine deaths using data from its own case work and freedom of information requests.
It says the figure may be higher as the NHS places nearly half of all its young psychiatric patients in private units which are not subject to FoI requests.
The Care Quality Commission, cited by the DoH as collecting accurate data, is only notified of the deaths of young people committed to hospital or sectioned. This omits the deaths of voluntary patients, who comprise a large part of the hospital population.
Inquest's director, Deborah Coles, said: "My fear is there could be more deaths. Neither we nor the Department of Health know."
Calling for statutory notification and an independent investigation when a child dies in a mental health unit, she added: "How on earth can we learn if we don't know the true picture and circumstances? If a child died in prison an independent investigation follows automatically.
"There isn't that independent scrutiny given to these deaths."
In light of the information being passed to Mr Burt, the DoH says NHS England now accepts there have been four deaths of young people "in an inpatient setting".
Mr Burt said: "Panorama's investigation has unearthed questions about record keeping and I'm seeking assurances from NHS England that they have the right processes in place for recording any such death and that lessons are learned."
Inquest analysed the responses to 238 Freedom of Information requests to NHS Trusts, Private Providers, local Safeguarding Children Boards, local councils, the Department of Education, the Department of Health and the Care Quality Commission.
Ms Coles concluded: "What's been really shocking is how difficult it is to find the true picture of the number of children dying in psychiatric care."
BBC Panorama will air on Monday at 7.30pm and will look at the case of Sara Green who took her own life at the Priory Hospital where she was an inpatient.
While at the unit, Sara tried to harm herself a number of times, and wrote in her diary that being in the unit made her feel worse.
Her mother, Jane Evans, told the programme Sara's last visit home was in March 2014, eight months after she was admitted to the Priory in Cheadle, Manchester.
"She didn't want to go back. I didn't want her to go back but I knew what would happen if she didn't. They would section her.
"And they'd made that quite clear over the period of time she was in there."
On March 18, 2014, the day after she returned to the Priory, Sarah died after self-harming.
Panorama reports that the coroner at Sara's inquest was heavily critical of the unit.
He concluded she did not intend to die, she was self-harming. Her anxiety having worsened by her unacceptably prolonged stay at the Priory, a hospital 100 miles from her home.
In response, The Priory said the safety of patients was its primary concern. A spokesman added: "Since this incident took place, we have conducted a full investigation to learn the lessons from what happened and have made a number of changes to our procedures, including improving record-keeping.
"We engaged fully with the coroner's Inquest and have implemented further changes identified by the coroner since then."