Staffing shortages could be putting mental health patients' safety at risk, inspectors have warned.
West London Mental Health NHS Trust (WLMHT), which serves more than 700,000 people, has been told it must buck up its ideas following an inspection by the Care Quality Commission (CQC).
A fresh inspection of the trust has seen it given a "requires improvement" rating by the watchdog, second only in its severity to an "inadequate" rating.
It was found wanting when it came to ensuring standards of "safety, effectiveness and leadership" in a report issued on Wednesday.
However, the trust, which is also responsible for Broadmoor Hospital in Berkshire, was graded "good" for being caring and responsive to patients' needs.
A team from the CQC, who spent five days inspecting the trust back in June, found the trust had a "substantial" problem with staff recruitment and retention.
They said there were too few staff to "consistently guarantee safety and quality" in its medium and low secure services, high secure services and community-based mental health teams.
Dr Paul Lelliott, CQC deputy chief inspector of hospitals and lead for mental health, said: "Staffing shortages and difficulties with recruitment at WLMHT are having a noticeable impact on the quality of some services.
"In the forensic services, this is compounded by low morale. As well as affecting the quality of care, there is the risk that staff might not show the openness, transparency and honesty that are essential to provide safe care."
Dr Lelliott added the team were also concerned by the apparent overuse of physical restraint, and failures to maintain proper record keeping.
"Staff must use restraint only as a last resort, and minimise the use of restraint in the prone (face-down) position," he said.
"They must record the use of all types of restrictive intervention. They must also make necessary physical health observations to ensure the safety of patients who have been given an injection to manage disturbed or distressed behaviour."
Inspectors found most staff were hard-working, caring and compassionate but that morale across the trust was low.
They praised the trust for encouraging personal development of staff and for working to improve engagement with service users and carers.
They also applauded it for working with partners in the statutory and voluntary sector to improve mental healthcare in the wider community, which they said had helped dramatically reduce the use of police cells to hold people detained under the Mental Health Act.
Steve Shrubb, WLMHT chief executive, described the assessment as "very fair".
"They recognised our staff's hard work and compassion," he said. "Many of our services were found to be good, and all of our services were found to be caring - staff should feel rightly proud of this.
"However, we know we have more work to do and need to deliver improvements if we are to deliver consistently high quality care to every single patient.
"I apologise to patients and their families if they have received poor care from us."
He added the trust had already made a number of changes following the inspection, which includes developing improvement plans for each service.
Progress will be monitored by the trust's NHS partners, as well as the CQC.