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.
Prime Minister Theresa May described events at Gosport as "deeply troubling" and apologised to families over the time it took to get answers from the NHS.
Health Secretary Jeremy Hunt told MPs police and the Crown Prosecution Service (CPS) would examine material in the report to consider their next steps and "whether criminal charges should now be brought".
Bridget Reeves - whose grandmother Elsie Divine, 88, died at the hospital in 1999 - said: "These horrifying, shameful, unforgivable actions need to be disclosed in a criminal court for a jury to decide and only then can we put our loved ones to rest."