All such deaths are supposed to be reported to a coroner so they can be investigated.
But official figures show that that of 1,115 cases recorded by the NHS over three years, just one in three were reported to coroners.
Charities said the figures were “alarming” and raised the risk that deaths were not being properly investigated, to reduce the risk of future tragedy.
Brian Dow, from the charity Rethink Mental Illness, said: “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.”
Official Ministry of Justice figures analysed by Health Service Journal 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 – 742 more than those reported to coroners.
Diane Abbott, shadow health secretary, said: “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.
A Department of Health spokesman said all deaths should be properly investigated, but said some discrepancies might be explained by different methods of recording.
A 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.”
Health watchdogs are currently examining NHS investigations into the deaths of patients, amid concern that the deaths of those with learning disabilities have not been properly probed.
The nationwide review came after Southern Health NHS Foundation Trust was criticised following the death of 18-year-old Connor Sparrowhawk.
Mr Sparrowhawk drowned in a bath at an Oxford mental health facility after suffering an epileptic fit in 2013.
Inspectors found the trust’s review of deaths was not robust enough.
The Care Quality Commission is inspecting 12 NHS trusts and said it would be “paying particular attention to investigations and and learning from deaths of people with a learning disability or mental health problem”.
An inquest into the death of Mr Sparrowhawk found that neglect from Southern Health staff contributed to his demise.
A report commissioned by NHS England suggested that hundreds of unexpected deaths at Southern Health were not investigated.