Washington (AFP) - A Texas judge on Wednesday refused to postpone the scheduled execution of a convicted killer who suffers from mental illness and is set to face lethal injection on December 3.

Scott Panetti, who has had schizophrenia for three decades, has won support for his case from groups like Mental Health America, psychiatrists, former judges and prosecutors and evangelical Christians.

The European Union has also urged Texas Governor Rick Perry to grant Panetti clemency.

"The execution of persons suffering from a mental disorder is contrary to widely accepted human rights norms and is in contradiction to the minimum standards of human rights set forth in several international human rights instruments," the bloc wrote in its letter.

Still, district judge Keith Williams refused to give attorneys more time to reevaluate whether Panetti was criminally responsible. He was convicted in 1995 of shooting his estranged wife's parents to death at point blank range in 1992.

At the trial, Panetti acted as his own attorney, wore a cowboy outfit and tried to call as witnesses the pope, John F. Kennedy and Jesus.

"As an obviously severely mentally ill man with schizophrenia, Mr Panetti should never have been allowed to represent himself in his death penalty case," his attorney Kathryn Kase said.

He "should not have been allowed to reject a plea deal that would have saved his life. Now, Mr Panetti must not be executed without a competency hearing.

"This is the last chance to prevent an injustice from turning into an immoral tragedy."

Though individual US states choose whether they will implement the death penalty, in 1986 the US Supreme Court barred execution of the mentally ill as cruel and unusual punishment.