A US District Court has blocked the execution of a schizophrenic man who had been sentenced to die in Texas for a double murder.

Scott Panetti, 65, who has suffered from mental illness for decades, was condemned to death in 1995 for the shooting of his estranged wife's parents.

US District Judge Robert Pitman, after a three-day evidentiary hearing which ended on Wednesday, said it is "undisputed that Panetti is severely mentally ill, suffering from chronic schizophrenia for over forty years.

"Panetti's mental illness prevents him from rationally understanding the state's reasons for executing him," Pitman said. "The court finds that Scott Panetti is not competent to be executed under the prohibition against cruel and unusual punishment set forth by the Eighth Amendment."

Gregory Wiercioch, Panetti's lawyer, welcomed the ruling, saying it "prevents the state of Texas from exacting vengeance on a person who suffers from a pervasive, severe form of schizophrenia."

"The Eighth Amendment bars the execution of people who, like Mr. Panetti, are severely mentally ill and do not understand the reason for their punishment," he said.

In September 1992, dressed in camouflage, Panetti broke into the house of the parents of his estranged wife and shot them dead.

He took his wife and three-year-old daughter hostage for the night before surrendering to police.

Panetti had been hospitalized more than a dozen times prior to the murders for hallucinations and psychotic episodes.

His long-running battle to stave off execution won support from the likes of the Mental Health America advocacy organization, psychiatrists, former judges, prosecutors and religious groups.

The case also garnered interest from the European Union, which urged the Texas authorities to grant Panetti clemency.

At trial, Panetti acted as his own attorney, wore a burgundy cowboy outfit and tried to call Pope John Paul II, John F. Kennedy and Jesus as witnesses.

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

There have been 18 executions in the United States this year. (AFP)