The deaths of hundreds of patients sectioned under the Mental Health Act may not have been investigated properly, it has emerged.
All deaths of people detained under the Act, even if due to natural causes, are supposed to be reported to a coroner, but figures show a discrepancy between the number of cases investigated by coroners and the number of deaths.
Official Ministry of Justice figures analysed by the Health Service Journal (HSJ) show that between 2011 and 2014, 373 deaths of people detained under the Act were reported to coroners in England and Wales.
But data supplied to the Independent Advisory Panel on Deaths in Custody show 1,115 deaths, HSJ found
Health officials said the discrepancy is likely to have occurred as a result of coroners not marking the deaths as under the Mental Health Act.
Brian Dow, director of external affairs at the charity Rethink Mental Illness, said: "These figures are alarming, especially if there is a risk that deaths have not been thoroughly investigated.
"If incidents are not being appropriately referred and examined then lessons can't be learnt about how to avoid further tragedies in the future.
"We owe it to people detained under the Mental Health Act and their families to ensure this.
"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."
A Department of Health spokesman 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."
Commenting on the figures, shadow health secretary Diane Abbott, said: "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
"It is outrageous that the Department of Health will not rule out that that some families have been deprived of an investigation. They deserve answers."