Diagnosing patients with medical labels such as "personality disorder" and "schizophrenia" can lead to less effective treatments being delivered, research has found.
Up to five people in every 100 are estimated to be diagnosed with personality disorders in the UK, with one to three in every 100 living with schizophrenia.
It is commonly believed that such diagnoses are stigmatising and lead to negative perceptions by the public, family members and those with mental health problems.
Previously, it had been assumed that those working in the mental health profession would be immune to such beliefs, but research by the University of Bath found that labelling patients with these conditions can have negative impacts on professionals and lead to less effective treatment.
The label "borderline personality disorder" was particularly problematic, as professionals could assume the patient was damaged in all areas of life and close contact should be avoided.
This might lead to clinicians wrongly assuming tendencies and behaviours from the patient, offering treatment for these instead of taking into account their individual needs.
The work is published in the British Journal of Clinical Psychology and Behavioural & Cognitive Psychotherapy.
Paul Salkovskis, professor of clinical psychology and applied science at the University of Bath, said the study showed labels led to professionals "judging a book by its cover".
"We know that therapists' expectations when they first assess patients will influence the later course of treatment," Prof Salkovskis said.
"This is why diagnostic labels can be so damaging for a patient as well as ineffective at treating the conditions they exhibit.
"Those working with patients with mental illness need to be extremely cautious both in the use of diagnostic labels to describe a patient and mindful of the influence that such labels can have on their own clinical judgments."
In a controlled experiment, the researchers used a video of a person suffering from a relatively simple anxiety problem before allocating mental health therapists into three groups.
Each group was given a different amount of "incidental" background information.
Before the video started, one group was given simple descriptions about a patient and another was given additional information corresponding to behaviours linked to personality disorders.
The third group was also told that a psychiatrist had previously suggested a diagnosis of personality disorder.
The therapists were then asked to watch the video and assess what they saw.
Researchers found those therapists who were given the diagnosis - which was false - were negatively influenced by it.
However, those who were given descriptions that corresponded to the diagnosis and those who were only given simple descriptions were not influenced.
This suggests the therapists were not influenced by the behaviour of the patient, only by the addition of the diagnosis, which made them inappropriately pessimistic.
Lorna Hogg, clinical tutor of the doctorate in clinical psychology programme at Bath, said: "What is particularly interesting about the findings is that evidence clearly indicates that having a personality disorder as well as anxiety doesn't make it any harder to treat the anxiety.
"So it is really important that clinicians stay up to date with current evidence and make all-important decisions about treatability based on this.
"Training and supervision are also really important to help clinicians become more aware of the judgments they make that might affect treatment and crucially challenge these."
The researchers are calling for greater awareness of the negative impacts such labels can have in the mental health profession.
Dr Danny CK Lam said: "The findings in these two research studies show that medical labels can be an obstacle not only to the treatment process and outcome, but can also be problematic to how the individual being labelled perceives him or herself as a person.
"The label is 'sticky' and stigmatising, hard to remove and is unlikely to be helpful to the individual's understanding of his mental health problem."