The firing squad is the only appropriate method of execution for a condemned Georgia prisoner - even though it is not permitted under state law - because the state's lethal injection drug could cause him to suffer more than the constitution allows, his lawyers say.

J W Ledford Jr is scheduled to be killed on Tuesday.

He was convicted of murder in the January 1992 stabbing of his neighbour, 73-year-old Dr Harry Johnston, near his home in Murray County, in northwest Georgia.

Ledford, 45, suffers from chronic nerve pain that has been treated with increasing doses of the prescription drug gabapentin for more than a decade, his lawyers said in a federal case filed on Thursday.

They cited experts who said long-term exposure to gabapentin alters brain chemistry, making pentobarbital unreliable to render him unconscious and devoid of sensation or feeling.

"Accordingly, there is a substantial risk that Mr Ledford will be aware and in agony as the pentobarbital attacks his respiratory system, depriving his brain, heart, and lungs of oxygen as he drowns in his own saliva," the legal case said.

That would violate the prohibition on cruel and unusual punishment in the Eighth Amendment of the US Constitution, Ledford's lawyers argued.

'Manipulative' case

But prosecutors on Friday called Ledford's legal case "manipulative". They said the experts cited by his lawyers made dubious claims based on speculation and that they have failed to show that pentobarbital is "sure or very likely" to cause him "a substantial risk of serious harm," as required by US Supreme Court precedent.

State lawyers also cited their own expert, Dr Jacqueline Martin, chief medical examiner for the Georgia Bureau of Investigation, who said the amount of pentobarbital used by the state is "more than sufficient" to carry out the execution without causing Ledford pain, despite his use of gabapentin.

The US Supreme Court has said that when claiming an execution method violates the Eighth Amendment, an inmate must propose a "known and available" method of execution.

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There is no alternative method of lethal injection available to the state, since the drugs used in executions have become harder for states to obtain because manufacturers have prohibited their use for capital punishment, the case said.

So Ledford's lawyers proposed using a firing squad, which the Supreme Court has already declared constitutional.

Georgia already has the skilled personnel, weapons and ammunition needed to carry out an execution by firing squad, Ledford's lawyers argued.

Three states - Mississippi, Oklahoma and Utah - allow for a firing squad as a backup if lethal injection drugs are not available, said Robert Dunham, executive director of the Death Penalty Information Center, which compiles capital punishment statistics.