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The controversial illness, multiple personality disorder, goes undiagnosed and untreated in the United Kingdom despite being present in about one in 14 patients who use psychological and psychiatric services, it was claimed this week.

The illness, in which sufferers develop several separate identities, is an extreme case of dissociative disorders, which are also heavily underdiagnosed despite affecting about one in eight of the same population, the British Psychological Society's clinical psychology annual meeting heard.

John Davis and Marcia Davis, clinical psychologists at the University of Warwick, screened local outpatients attending three services in the area (psychiatry, clinical psychology and psychotherapy). They used questionnaire and follow-up interviews. They found that 15 per cent of the patients fitted their definition of dissociative disorders - which are characterised by symptoms such as trances and amnesia. The symptoms are believed to be the result of sufferers distancing themselves from traumatic experiences they have had often in childhood.

Dr John Davis said that the disorders are taken more seriously in the United States. But in the UK sufferers are categorised as having other illnesses.

But Peter Raven, psychiatrist and lecturer in affective disorders at the Institute of Psychiatry, said that most psychiatrists would disagree with a diagnosis of multiple personality disorder. "It's controversial. Most psychiatrists would not feel any particular need for the diagnosis. He said that rigour in diagnosis was crucial: "The questions you ask often determine the answers you get. There are no internationally agreed definitions about what constitutes multiple personality disorder."

He said some people believe the disorder is induced by therapy: "The suggestion is that by asking about different facets of a person's behaviour and by planting different seeds of ideas you can induce this fragmented personality."