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Horrific crimes, such as the Anders Breivik case, illustrate the misconceptions the public has about mental illness, a leading expert says.

Professor Simon Wessely, of King's College London, said the simplest responses to mass killings were that the perpetrators "must be mad".

But he said the way Breivik carried out the killings suggested otherwise.

He said the idea a psychiatric diagnosis could help people avoid punishment was wrong too.

Writing in the Lancet medical journal, Professor Wessely said putting forward a mental illness defence in the UK could lead a person to spending more time behind bars than less.

"The forensic psychiatry system is not a soft or popular option," he added.

The psychiatrist also said the Breivik case highlighted another misconception - that outrageous crimes must mean mental illness.

"For schizophrenia to explain Breivik's actions, they would have to be the result of delusions."

But he added: "The meticulous way in which he planned his attacks does not speak to the disorganisation of schizophrenia."

Breivik is currently on trial in Norway.

The 33-year-old admits to killing 77 people in Oslo and on Utoeya island last July but denies criminal responsibility.

Two reports have been compiled on his mental state and have come to opposing views on his sanity.

The court's ruling on this will determine whether Breivik is sent to jail or into psychiatric care.

Breivik has argued he is not insane and should either be put to death or acquitted.