By David Beasley

(Reuters) - The U.S. Supreme Court on Thursday ordered a last-minute hold on the Alabama execution of Thomas Douglas Arthur, the seventh time he has avoided capital punishment for the 1982 murder of his girlfriend's husband.

Arthur, now 74, has been on death row for more than three decades having been convicted of shooting to death Troy Wicker as he slept, court records showed. Prosecutors said Arthur's girlfriend, Judy Wicker, paid him $10,000 to kill her husband.

Alabama had sought to execute Arthur despite questions about its death penalty process following a U.S. Supreme Court ruling in January that struck down a Florida law giving judges powers that juries should wield in deciding death eligibility.

The U.S. Supreme Court has since ordered Alabama to review similar practices in four other cases.

"Upon consideration of the application of counsel for the applicant, it is ordered that execution of the sentence of death is hereby stayed pending further order of the undersigned or of the court," stated the one-page order signed by Justice Clarence Thomas.

The order did not state a reason for the stay of execution.

Arthur's attorneys have challenged the constitutionality of Alabama's lethal injection method of execution on the grounds that it is cruel and unusual punishment.

In their challenge, his lawyers claimed that midazolam, the first drug that is used in executions causes "excruciatingly painful and agonizing effects of the second and third drugs.”

Arthur's scheduled execution followed three trials and another man's confession to the crime. In all, Alabama had scheduled Arthur's execution six times before Thursday.

He had been scheduled to die on Thursday evening at the Holman Correctional Facility in Atmore. He would have been this year's 18th U.S. execution and the state's second, the Death Penalty Information Center said.

Arthur had two convictions overturned on constitutional grounds, including improper introduction of evidence about a prior murder conviction. After his third conviction in 1991, he asked the jury to sentence him to death, seeking more time with his children during prison visits and a private cell.

The killing of Troy Wicker in Muscle Shoals, Alabama, occurred when Arthur was in a prison work release program after an earlier murder.

Judy Wicker told police a black man raped her, knocked her unconscious and shot her husband at their home. Arthur, who is white, disguised himself as a black man, prosecutors said.

At her trial, Judy Wicker denied Arthur was the killer but later changed her testimony during his trial, Arthur's lawyers said. She was convicted of murder and paroled after 10 years in prison, according to the Alabama Department of Corrections.

In 2008, another inmate, Bobby Ray Gilbert, confessed to killing Wicker but a state court held that Gilbert and Arthur had conspired to submit a fake confession.

Limited crime scene testing found no DNA link to Gilbert or Arthur. Alabama lost a rape kit that might have cleared Arthur, his lawyers said.

(Reporting by David Beasley in Atlanta and Alex Dobuzinskis; Editing by Grant McCool, Bill Trott, Simon Cameron-Moore and Michael Perry)