Norfolk and Suffolk's NHS mental health services are preparing to fully merge.
The regulatory body Monitor said Norfolk & Waveney Mental Health's acquisition of the Suffolk Mental Health Partnership would be "in patients' interests".
Norfolk & Waveney had already been put in interim charge of the Suffolk trust.
Dr David Bennett, chair of Monitor, said: "We've made this decision based on evidence there are pressing clinical quality and safety issues at Suffolk."
A review of patient safety called the Rae Report was published in January following the death of a 19-year-old man on the Southgate Ward in Bury St Edmunds in August 2010.
The report, which also looked at five other deaths between May and August 2009, was critical of some aspects of management in Suffolk.
Dr Bennett said: "The board therefore decided that it would be in patients' interests if the acquisition were to go ahead so that these issues can be addressed quickly and effectively."
Dr Hadrian Ball, medical director of Norfolk & Waveney Mental Health and interim medical director of Suffolk Mental Health, said: "You need the right management and governance structures and what happened was the structures [at Suffolk] were not what they could have been.
"The primary argument for going ahead with this merger was the need to ensure that the improvements in clinical services that have been brought about recently are maintained and are further embedded.
"Patients will not immediately notice much at all in terms of direct delivery of care, which is as good as it will be anywhere else."
Norfolk Chief Executive Aidan Thomas, chief executive of the Norfolk & Waveney NHS Foundation Trust, said: "The trust was asked to take over the management of Suffolk Mental Health Partnership Trust by the NHS East of England Strategic Health Authority, as a result of serious safety concerns.
"Throughout the last six months, Suffolk Trust has been managed as a separate entity, with its own board and its own structures, in order to avoid pre-empting any decision about the merger."
Once the acquisition has gone through, the merged service will be called the Norfolk & Suffolk NHS Foundation Trust.