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Research from Norfolk and Suffolk NHS Foundation Trust and University of East Anglia suggests patients would prefer to be told their diagnosis face-to-face rather than by other means.
However, doctors said that a diagnosis can be complex and take time to assess.
Royal College of GPs chairwoman Prof Helen Stokes-Lampard told the BBC a diagnosis should be a "starting point".
She said it should be the position "from which we can begin a course of treatment tailored to that individual, in partnership with them. No patient should ever see a diagnosis as a 'meaningless label'."
Isolation and confusion
Nearly a third of more than 2,220 people questioned for the study, in The Lancet Psychiatry, were from the UK.
Participants also took part in 12 other countries, with most of the rest in the USA, Australia and Canada.
Dr Corinna Hackmann, one of the report's authors, told the BBC: "Receiving a mental health diagnosis can be very helpful but it can also be devastating."
She said the findings suggested doctors could make assumptions about what they believe to be in patients' best interests by, "for example, sometimes delaying or withholding diagnosis due to concern over the impact it might have on the person".
However, she said the data suggested that patients "would prefer to be involved and informed in the whole process of mental health diagnosis and decisions regarding how their diagnosis relates to treatment".
"Going forward, ideally the diagnostic process would be collaborative and respond to individual need in a way that explains difficulties, gives hope and guides recovery," she said.
The research found evidence of feelings of isolation, confusion, insignificance or distress when a diagnosis was not discussed with patients, but instead was revealed via health records, in a letter or mentioned accidentally in a care meeting.
It suggests that patients want to be told of their diagnosis personally so that they can discuss care needs and treatment.
However, doctors said that diagnosis was not always straightforward and could take time. Issues include overlapping symptoms and decisions on how significant some of them were.
From the patients' point of view, they felt that a delayed diagnosis promoted feelings of rejection and they more often reported a positive experience when they felt their diagnosis was efficient and timely.
Dr Caitlin Notley, of the UEA's Norwich Medical School, said: "This research supports a sensitive, individualised, collaborative, and holistic approach to mental health diagnosis, which is informative, can empower service users, provide hope and guide treatment."
'Very useful' research
Prof Stokes-Lampard said: "As GPs, we are in a unique position in the health service to deliver holistic person-centred care, and part of this is being able to talk to patients face-to-face in the GP consultation.
"We will always strive to deliver significant diagnoses in person rather than remotely, particularly for mental health conditions, to give the patient the opportunity to ask questions and for us to properly discuss the potential risks and benefits of any treatment plan in a way that is tailored to the needs of the patient.
"The NHS, including general practice, relies on patient feedback to help improve services and this research is very useful."
Royal College of Psychiatrists spokesman Prof Sukhi Shergill told the BBC: "It comes as no surprise that patients can be left feeling confused and upset if their diagnosis is not properly explained to them.
"Diagnosis is a helpful tool when applied according to International Classification of Diseases guidelines. But it is just a tool - what really matters is how it is used in the interaction between the patient, psychiatrist and carers.
"It is crucial that it is communicated to the patient in a way which is clear, sensitive and offers hope."