A federal judge has ruled that Tennessee's new lethal injection procedures are cruel and unusual punishment, interrupting plans to execute a killer next week.

The protocol "presents a substantial risk of unnecessary pain" and violates death row inmate Edward Jerome Harbison's constitutional protections under the Eighth Amendment to the U.S. Constitution, U.S. District Judge Aleta Trauger said Wednesday.

The new protocol, released in April, does not ensure that inmates are properly anesthetized before the lethal injection is administered, Trauger said, which could "result in a terrifying, excruciating death."

A spokeswoman for the state attorney general's office said officials are reviewing the ruling and have not decided whether to appeal. Gov. Phil Bredesen's office had no immediate comment.

Harbison was scheduled to be executed Sept. 26 for beating an elderly woman to death during a burglary in 1983.

Trauger did not issue a stay or throw out the death sentence for Harbison, who has lost all his appeals. He can be legally executed once the state adopts a valid method of execution, she said.

A nonprofit group that supports the death penalty predicted the appeals court could reverse Trauger's ruling.

"They have been fairly hostile to these sorts of claims," said Kent Scheidegger, legal director of the Criminal Justice Legal Foundation.

But an organization that opposes capital punishment noted that Wednesday's ruling could have wider effects.

"Other states are reviewing this process, but there have been few clear rulings," said Richard Dieter, executive director of the Death Penalty Information Center. "This is a significant precedent, and we'll see what happens in other states."

Bredesen in February placed a 90-day moratorium on executions because of several glaring problems with the state's execution guidelines, including conflicting instructions that mixed lethal injection instructions with those for the electric chair.

George Little, State Department of Correction commissioner, adopted the new protocol despite having knowledge about the remaining risks of excessive pain for inmates, Trauger said. A spokeswoman for Little did not immediately return a message seeking comment.

Little did not give enough consideration to a recommendation to discard the standard three-drug lethal injection cocktail in favor of a single drug method, Trauger said. Current training and medical expertise are not sufficient to ensure a painless execution, she said.

Most states use three drugs _ thiopental, an anesthetic; pancuronium bromide, a nerve blocker and muscle paralyzer; and potassium chloride, a drug to stop the heart. Each is supposed to be capable of killing by itself, but if not, the anesthetic is supposed to render the inmate unconscious while the other drugs do the job.

Lethal injection has been adopted by 37 U.S. states as a cheaper and more humane alternative to electrocution, gas chambers and other execution methods. But at least 11 states suspended its use after opponents alleged it was ineffective and cruel.

The issue came to a head last year in California when a federal judge ordered that doctors assist in killing Michael Morales, who was convicted of raping and murdering a teenage girl. Doctors refused, and legal arguments continue.

The 8th U.S. Circuit Court of Appeals last month lifted a more than year-old stay on executions in Missouri, refusing to block capital punishment while a death-row inmate asked the U.S. Supreme Court to declare the state's form of lethal injection to be an unconstitutionally cruel punishment.