A federal appeals court has denied condemned killer Warren Lee Hill one of his last legal recourses to avoid execution.

The 11th U.S. Court of Appeals in Atlanta declined Tuesday to review a motion to delay Hill’s execution until the U.S. Supreme Court can consider the merits of new evidence his lawyers claim show Hill is mentally retarded and ineligible for the death penalty. Hill is slated to be executed Monday.

Hill’s lawyers have appealed to the U.S. Supreme Court to grant the stay until the Court can decide whether to take on the case, currently scheduled for consideration Sept. 30.

» MORE: The Warren Hill case so far from the AJC's Rhonda Cook

If the Supreme Court decides to grant the stay, it can either decide to consider it in September or transfer it to a federal district court in Georgia. Or, the Court can deny the stay and the petition for appeal, and Hill will be executed on Monday.

“I think we have a very meritorius claim,” said Hill’s laywer, Brian Kammer. “It is a very unusual set of circumnstances and I think it warrants the Supreme Court’s intervention.”

The U.S. Supreme Court determined in 2002 executing mentally retarded inmates to be unconstitutional. But in Georgia, intellectual disability must be proven “beyond a reasonable doubt,” one of the strictest standards of its kind in the nation.

Hill was already serving a life sentence for murdering his 18-year-old girlfriend when, in 1990, he killed fellow inmate John Handspike at the Lee State Prison in South Georgia. Hill beat to death his inmate with a nail-studded board, prosecutors said.

Earlier this year, the three experts the state used to prove Hill was competent to stand trial came forward to say they were mistaken. The doctors — two psychiatrists and a psychologist — had previously testified that Hill was faking his mental disabilities. They described their evaluations of Hill more than a decade ago as rush jobs.

The 11th Circuit granted him a stay in February until the new evidence for Hill’s mental disability could be considered. The news came less than an hour before Hill was to be put to death.

Then, in April, the court decided that Hill’s mental retardation claim was the same it had already rejected, despite the changed opinions of doctors and other mental health experts who now say Hill is incompetent to stand trial.

“If all that was required to reassert years later a previously rejected claim was a change in testimony, every material witness would have the power to upset every notion of finality, ” 11th Circuit Judge Frank Hull wrote in the court’s majority opinion.

In a strongly worded dissent, Judge Rosemary Barkett wrote there is now “no question” that Georgia will be executing a mentally retarded man.

If the U.S. Supreme Court does not grant the stay, Hill will be executed Monday at 7 p.m. at the Georgia Diagnostic and Classification Prison at Jackson, 50 miles south of Atlanta.