More than 450 patients died after being given powerful painkillers inappropriately at Gosport War Memorial Hospital, a report has found.
An independent panel said, taking into account missing records, a further 200 patients may have suffered a similar fate.
The report found there was a “disregard for human life” of a large number of patients from 1989 to 2000.
It said Dr Jane Barton oversaw the “practice of prescribing” on the wards.
There was an “institutionalised regime” of prescribing and administering “dangerous” amounts of a medication not clinically justified at the Hampshire hospital, the report said.
Former Bishop of Liverpool James Jones, who led Gosport Independent Panel, said: “The documents seen by the panel show that for a 12-year period a clinical assistant, Dr Barton, was responsible for the practice of prescribing which prevailed on the wards.
“Although the consultants were not involved directly in treating patients on the wards, the medical records show that they were aware of how drugs were prescribed and administered but did not intervene to stop the practice.”
Relatives had said they hoped the findings of the report would end their “harrowing” wait for answers.
The report said families were “consistently let down” by those in authority, both medical individuals and institutions, when they complained about the treatment of their loved ones.
So far, the only person to face disciplinary action has been Dr Barton, who was found guilty of failings in her care of 12 patients at Gosport between 1996 and 1999.
But no prosecutions were brought and she was not struck off the medical register, choosing to retire after the findings.
Bishop Jones, who also headed the Hillsborough inquiry, said: “Its is not for the panel to ascribe criminal or civil liability.
“It will be for any future judicial processes to determine whatever culpability and criticism might be forthcoming.”
Police previously investigated the deaths of 92 patients during three inquires between 1998 and 2006, but no prosecutions were brought.
Chief Constable Olivia Pinkney, of Hampshire Constabulary, said the force had “co-operated fully with the panel and “shared with them more than 25,000 documents containing 100,000 pages of information”.