Inspectors have issued a warning notice to an NHS trust where an 18-year-old drowned in a bath, saying 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 that it 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.
Dr Paul Lelliott, CQC deputy chief inspector of hospitals and lead for mental health, added: "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.
Dan Scorer, head of policy at learning disability charity Mencap, said the CQC warning notice states that little has changed since the independent report was published.
"Families are being left questioning whether the death of their loved one should have been investigated and whether the death might have been avoided," he said.
"Whilst Mazars (report) exposed the failures to investigate deaths at Southern Health, we have known since 2013 that 1,200 people with a learning disability die avoidably in our NHS every year. The lack of urgency to tackle this national scandal is unacceptable."
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."
Luciana Berger, Labour's shadow minister for mental health, said she would raise the issue when Parliament resumes next week, adding: "It is extremely worrying that the trust's leadership has not taken the appropriate action to improve patient safety."
A Department of Health spokeswoman said: "The Mazars report into Southern Health clearly outlined wholly unacceptable failings and work is under way to ensure lessons are learnt both by the trust and across the system as a whole."