Hundreds of patients who died whilst detained under the Mental Health Act may have been denied inquests, according to a report.
Coroners are supposed to be notified of all such cases, but official figures show that two thirds of the 1.115 deaths recorded by the NHS over three years to 2014 went unreported.
Charities have criticised the “alarming” statistics and say urgent action is needed to safeguard patients in the future.
The government data was obtained from the Ministry of Justice and the Independent Advisory Panel on deaths in custody.
Diane Abbott, shadow health secretary, told the Daily Telegraph: “These figures are shocking. If the state has deprived someone of their liberty and they then die under detention, their death must be reported to a coroner. If you are not learning about what is causing the deaths, you are limiting the ability to learn for the future.
“What is more alarming is that if this data is accurate, only a minority of deaths in state detention have been investigated by a coroner. These are deaths of people who are owed a duty of care by the state.”
Brian Dow, from the charity Rethink Mental Illness, added: “If incidents are not being appropriately referred and examined, then lessons can’t be learnt about how to avoid further tragedies.
“We want to see a robust, independent and transparent system for investigating deaths in mental health settings so no more families are left without answers.”
The Care Quality Commission (CQC) is inspecting 12 NHS trusts and said it would be "paying particular attention to investigations and learning from deaths of people with a learning disability or mental health problem".
A spokesman for the Department of Health said: “Families deserve an explanation if their loved ones pass away under NHS care and we expect every death in detention to be investigated thoroughly to make sure lessons are learnt.
“The Care Quality Commission is reviewing the quality and robustness of NHS investigations into deaths under the Mental Health Act; however, there is no evidence of significant under-reporting.”