The Care Quality Commission (CQC) has ranked 39 of the 59 NHS trusts that provide mental health services in England using evidence including concerns raised by healthcare staff, bed occupancy rates, and staff and patient surveys.
It placed Central and North West London NHS Foundation Trust in its highest risk band, together with only three other trusts.
Dr Katherine Rake OBE, chief executive of Healthwatch England, said immediate action was needed at all four trusts.
She added: “We hope that armed with this new intelligence, the inspectors will be able to target their efforts. It is critical that standards are driven up to ensure everyone has access to the same high quality care.”
The trusts were ranked into tiers, from Band 1, representing the highest risk, to Band 4. The findings have been published on the CQC's website.
The CQC has stressed that its findings are not judgements, but will be used to prioritise those trusts most in need of inspection.
Four 'risks' and two 'elevated risks' were identified at the West London trust.
The elevated risks covered the proportion of patients receiving physical health checks on admission and the level of planning ahead of patient discharges.
The risks covered the number of deaths of patients detained under the Mental Health Act, the number of reports written by approved mental health practitioners, a governance risk rating, and fully and partly upheld investigations into complaints.
However, the trust said one of the indicators used in the assessment, a Monitor Governance risk rating, was out of date, and that had the correct rating been used, it would have been placed in Band 2.
Regardless, it said it would use the CQC's assessment to identify areas in need of improvement.
Chief executive Claire Murdoch said: “We are disappointed at the overall rating and are looking forward to our (already announced) CQC Inspection in February, which is a week-long and thorough inspection – something we very much welcome.
“We hope this will allow the inspectors to test the reality of the standards of care we actually provide. What is important to us is that every patient receives high quality care and feels they have had the best possible experience.”
This week, the CQC also published similar rankings of nearly all GP surgeries in England, which showed that a quarter of practices in the borough could be putting patients at risk.