Inspectors have issued a warning notice to an NHS trust where an 18-year-old drowned in a bath, telling it it must urgently improve.
The Care Quality Commission (CQC) has told Southern Health NHS Foundation Trust it must make significant improvements to "protect patients who are at risk of harm" and had failed to learn from previous mistakes.
In October, a jury inquest ruled that neglect contributed to the death of Connor Sparrowhawk, who drowned after an epileptic seizure at Slade House, in Headington, Oxfordshire, in 2013.
The learning disability unit, run by Southern, has since closed its doors.
In a statement, the CQC said it has issued a warning notice "requiring the trust to improve its governance arrangements to ensure robust investigation and learning from incidents and deaths, to reduce future risks to patients".
CQC inspectors visited the trust in January as part of an inspection.
This followed the publication of an independent report commissioned by NHS England which said the trust had failed to investigate and learn from the deaths of patients.
The CQC statement said: "Inspectors found that the trust had failed to mitigate against significant risks posed by some of the physical environments from which it delivered mental health and learning disability services and did not operate effective governance arrangements to ensure robust investigation of incidents, including deaths.
"It did not adequately ensure it learned from incidents to reduce future risks to patients. In addition, inspectors found that the trust did not effectively respond to concerns about safety raised by patients, their carers and staff, or respond to concerns raised by trust staff about their ability to carry out their roles effectively."
Dr Paul Lelliott, CQC deputy chief inspector of hospitals and lead for mental health, added: "We have made it clear that the safety of patients with mental ill health and or learning disabilities provided by Southern Health NHS Foundation Trust requires significant improvement.
"We found long-standing risks to patients, arising from the physical environment, that had not been dealt with effectively. The trust's internal governance arrangements to learn from serious incidents or investigations were not good enough, meaning that opportunities to minimise further risks to patients were lost.
"It is only now, following our latest inspection, and in response to the warning notice, that the trust has taken action and has identified further action that it will take to improve safety at Kingsley ward, Melbury Lodge and Evenlode in Buckinghamshire."
The CQC will publish its full report in late April.
NHS Improvement said it intends to take further regulatory action at the trust to ensure urgent patient safety improvements are made.
It will put an additional condition in the trust's licence to provide NHS services, which means it could make changes to the management.
Dr Kathy McLean, executive medical director at NHS Improvement, said: "Patients and service users at Southern Health expect to get safe and good quality care, and it is worrying to see that the CQC have identified patient safety concerns which have still gone unaddressed at the trust.
"The trust needs to ensure that it fixes these issues quickly and that it can spot and quickly mitigate any future risks to patients and service users. If we don't see enough progress on this we will consider taking action on behalf of patients."
Katrina Percy, chief executive of Southern Health, said: "I have been very clear and open that we have a lot of work to do to fully address recent concerns raised about the trust.
"Good progress has been made, however we accept that the CQC feels that in some areas we have not acted swiftly enough. My main priority is, and always has been, the safety of our patients. We take the CQC's concerns extremely seriously and have taken a number of further actions.
"I want to reassure our patients and their families that I, and the board, remain completely focused on tackling these concerns as quickly as possible."