AUSTIN, TEX. ? The Supreme Court’s ban on executing mentally retarded killers has prosecutors across the country bracing for appeals from inmates hoping to avoid the death chamber.

“All of a sudden everybody on death row is going to become retarded,” said Mississippi Assistant Atty. Gen. Marvin “Sonny” White, who prosecutes death row appeals for the state. “It means new rounds of appeals.”

The high court ruled 6-3 on Thursday that executing mentally retarded murderers was unconstitutionally cruel. The decision the biggest shift in the court’s stance on capital punishment in a quarter-century offers the possibility of reprieve to scores of inmates.

Louisiana Atty. Gen. Richard Ieyoub said the ruling created a dilemma because the court left it to the states to legally define mental retardation. Those definitions would likely be tested in court, he said.

As states scrambled to figure out how the ruling would affect them, death-penalty opponents lauded Thursday’s opinion and victims’ advocates criticized it.

“It’s a Pandora’s box, and it’s deep and wide,” said Dianne Clements, president of Justice For All, a Texas victims’ rights group. Her son was murdered in 1991.

Texas Atty. Gen. John Cornyn declined to comment Thursday, but the ruling was already making waves in the nation’s leading death penalty state.

Hours after the ruling, lawyers for convicted killer John Paul Penry, who is considered mentally retarded, asked for a mistrial. The judge denied the motion and continued a hearing to determine whether Penry should be sentenced to death a third time for a rape-slaying that happened more than 22 years ago.

Eighteen of the 38 states that allow the death penalty exempt mentally retarded people. Twelve states and the District of Columbia do not impose the death penalty. There are more than 3,700 death-row inmates nationwide.

The Death Penalty Information Center claims Texas has executed at least six mentally retarded inmates since 1982.