A paranoid schizophrenic who stabbed to death a police officer had not been "fully well at any time" for the previous ten years, a jury was told.
Consultant psychiatrist Dr James Collins told Birmingham Crown Court that Glaister Earl Butler had displayed symptoms of the condition since 1994 but it may well have been developing since the mid-1980s.
Butler (49) denies murdering Detective Constable Michael Swindells in Birmingham on May 21 last year but admits manslaughter due to diminished responsibility.
Dr Collins, who works at Ashworth high security hospital in Liverpool, told Courtenay Griffiths QC, for the defence: "I don't think he's been fully well at any time during (in the last) ten years.
"The diagnostic features (of paranoid schizophrenia) are hallucinations and paranoid delusions. I think at times, particularly in the early period after his third discharge (from detention under the Mental Health Act), he might not have been experiencing these core features.
"But I think other features of the illness - social isolation, his lack of insight (into his condition) - have all been present. I don't think he's ever fully recovered.
"At the severe end of the spectrum, schizophrenia can be described as waxing and waning, ebbing and flowing. I think Mr Butler has exacerbations and periods when he's been severely unwell but I don't think he's had a period when he's not been ill at all."
The court has been told that Butler's mental health began deteriorating when he was made redundant from Rolls-Royce in 1982.
His paranoid delusions involved MI5, the police and security services as well as persecution by white people and the Government.
Asked about his mental state at the time of the killing, Dr Collins said: "Given the nature of his delusional system, that is with the police, it is hard to imagine that his behaviour at the time was not significantly affected by the illness.
"In my experience, a case such as this would meet the verdict of diminished responsibility."
The trial continues.