A Florida judge has ruled that a diagnosed paranoid schizophrenic who believes he is the "Prince of God" and is convinced that he will be resurrected to sit for eternity at God's right hand is sane and can put to death next week.

David Glant, a judge with Florida's eighth circuit, has found that John Ferguson, 64, can be given lethal injections next Thursday despite a US supreme court injunction that prohibits executions of the insane. In his concluding remarks, the judge agrees with the prisoner's defence lawyers that Ferguson is a paranoid schizophrenic who genuinely believes he is "Prince of God".

The judge accepts that the prisoner has a "long history of mental illness" and finds there is no evidence to support the prosecution contention, made over several years, that Ferguson was pretending to be mentally ill in order to avoid execution. Yet he goes on to conclude that "there is no evidence that his mental illness interferes, in any way, with his 'rational understanding' of the fact of his pending execution and the reason for it".

Astoundingly, the judge goes on to say that the prisoner's "grandiose delusion" of himself as being akin to Jesus at the point of his resurrection is in fact "relatively normal Christian belief". He writes: "There is no evidence that Ferguson's belief as to his role in the world and what may happen to him in the afterlife is so significantly different from beliefs other Christians may hold so as to consider it a sign of insanity."

Ferguson's legal team has appealed the ruling to the Florida supreme court and an initial hearing is likely to be held on Monday. Pending a final appeal to the US supreme court, the highest judicial panel in the land, the prisoner he will face execution by 4pm on Thursday.

"This is a guy with long-standing mental health issues that stretch over four decades, with 30 different doctors having diagnosed him as a paranoid schizophrenic, including Florida state and court-appointed doctors. And yet he is sane enough to be executed?" said Ben Lewis, one of Ferguson's defence lawyers.

Ferguson was sentenced to death in 1978 for multiple murders. In July 1977, together with two other men, he was involved in the shooting of six men after breaking into a house in Carol City; the following year on his own he murdered two teenagers in Hialeah.

Despite the undoubtedly harrowingly gruesome nature of his crimes, legally that should have no bearing on the current calculation as to whether he is mentally competent to be put to death. In 1986, in a case that coincidentally also arose in Florida, the supreme court ordered that under the eighth amendment of the US constitution, that bans cruel and unusual punishment, the death penalty could not be meted out to the insane.

The judgment said: "Whether the aim is to protect the condemned from fear and pain without comfort of understanding, or to protect the dignity of society itself from the barbarity of exacting mindless vengeance, the restriction finds enforcement in the eighth amendment."

The earliest citing of Ferguson's mental health problems goes all the way back to 1965 when he was reported as having had visual hallucinations. He was frequently confined to hospital for psychosis and schizophrenia through the 1970s.

In 1975 a doctor wrote that he had "a long-standing, severe illness which will most likely require long-term inpatient hospitalisation. This man is dangerous and cannot be released under any circumstances."

He was released, and two years later committed the first of his murders.

Over the years Ferguson has regularly experienced hallucinations. He hears his dead father talking to him, believes that people are listening to him and at one point reported that cockroaches had entered his brain through his ears.

Before this week's court hearing into Ferguson's state of mind, he was examined by a panel of three state-appointed psychiatrists. They interviewed him for just 90 minutes and on the basis of that found with "reasonable medical certainty that Mr Ferguson has no genuine current mental illness and understands the nature and effect of the death penalty and why it was imposed on him".

In his report, Glant conceded that state-appointed doctors "did not complete a thorough and exhaustive interview of Ferguson at Florida state prison". But then, the judge added, "That was not their mandate".