 The Trial of Lunatics Act 1883 was the next development in the law , allowing the jury to return a verdict that the defendant was guilty , but insane at the time , and should be kept in custody as a " criminal lunatic " . This Act was passed at the request of Queen Victoria , who , the target of frequent attacks by mentally ill individuals , demanded that the verdict be changed from " not guilty " so as to act as a deterrent to other lunatics ; the phrasing of " guilty of the act or omission charged , but insane so as not to be responsible , according to law , for his actions . " remained in use until the Criminal Procedure ( Insanity ) Act 1964 . 
