West London Mental Health NHS trust is to be investigated by the Healthcare Commission following concerns about how it handles safety incidents.
These incidents include cases of people self-harming.
The investigation will examine whether the trust takes appropriate action to address the causes of incidents in order to prevent them reoccurring.
Unannounced visits by the commission's investigators in February provided enough concern for a full investigation to make sure there are adequate systems and processes in place to protect the safety of service users.
Investigators will look at all incidents since April 2005, how they were dealt with and if the trust learnt from them to improve services.
West London Mental Health NHS trust employs 3,800 staff across 32 sites and provides care for up to 18,000 patients at any one time.
It is responsible for high security services at Broadmoor hospital but these are not the main focus of the investigation.
Nigel Ellis, the commission's head of investigations, said: "Clearly, there
are particularly challenging safety issues that have to be managed at
mental health trusts.
"It is crucial that every trust monitors and manages risks in order to protect the safety of patients."
He added: "We are not saying the services provided by the trust are unsafe.
"If we believed they were unsafe we would take immediate action. But we have a
duty to patients to be certain that all necessary systems are in place to manage risks.
"We are pleased to have the full cooperation of the trust in our work and will report back fully and publicly in due course, making recommendations to improve services if necessary."
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