UPDATE 1-Top U.S. court to rule on lethal injections

(Updates throughout, adds byline)

By James Vicini

WASHINGTON, Sept 25 (Reuters) - The U.S. Supreme Court said on Tuesday it would decide whether the commonly used lethal injection method of execution for death row inmates violated the constitutional ban on cruel and unusual punishment.

The nation's highest court said it would decide an appeal by two death row inmates from Kentucky arguing that the three-chemical cocktail used in lethal injections inflicted unnecessary pain and suffering.

Their lawyers said the Supreme Court has not addressed the constitutionality of a method of execution or the legal standard used to determine whether it is an unconstitutional form of cruel and unusual punishment in more than 100 years.

The last ruling occurred in 1878, when the Supreme Court upheld the constitutionality of the firing squad, they said.

The new case, which could limit or condone current forms of execution, will be one of the most closely watched of the Supreme Court's new term that begins on Oct. 1.

Another key case involves the rights of prisoners at Guantanamo Bay.

All but one of the 38 U.S. states with the death penalty and the federal government use lethal injection for executions. The only exception is Nebraska, which requires electrocution.

Ohio State University law professor Douglas Berman said the case could be pivotal.

"This is huge news which could (and probably should) lead to a de facto moratorium on all lethal injection executions nationwide until the Supreme Court issues a ruling," Berman said on his online Web site, "Sentencing Law and Policy."

Deborah Denno, a Fordham University law professor, said the lethal injection issue "is one of the foremost factors affecting the number of executions in this country. It has trumped just about every other issue."

The standard method of lethal injection, used by every state but New Jersey, involves administering three separate chemicals: sodium pentothal, an anesthetic, which makes the inmate unconscious; pancuronium bromide, which paralyzes all muscles except the heart; and then potassium chloride, which stops the heart, causing death.

HALF OF INMATES CHALLENGED LETHAL INJECTION

Attorneys for the two Kentucky men said at least half of the death row inmates facing imminent execution in the last two years have filed suit challenging the chemicals used in lethal injections.

While various lower court federal judges have ruled on the issue, the Supreme Court has yet to decide it. The case is expected to be argued early next year, with a decision likely by the end of June.