WASHINGTON (AFP) - A woman convicted of the brutal murder of a mentally disabled man is poised to become only the 14th female to be executed in the United States since 1976 on Wednesday when she faces death by lethal injection in Texas.

Barring a successful last-ditch appeal, Suzanne Basso, 59, will be executed by lethal injection at 6pm local time (8am Thursday Singapore time) at Huntsville Prison in Texas.

The wheelchair-bound killer, who weighed 160kg at the time of her arrest in September 1999, was sentenced to die for the horrific killing of Louis "Buddy" Musso" in 1998, whom she killed in an attempt to benefit from his life insurance.

Mr Musso was burned with cigarettes and beaten with belts, baseball bats and hobnailed boots by Basso and five others. His body, bloodied and battered beyond recognition, was found dumped in a roadside ditch in Houston in August 1998, according to court documents.

Despite a series of appeals which led all the way to the US Supreme Court, Basso's death sentence has been reconfirmed over the years, meaning she will be executed on Wednesday unless the Texas board of parole decides otherwise.

Anti-death penalty advocates have condemned Basso's looming execution, however, on the grounds that her physical and psychological state - she has shown signs of suffering from mental illness - mean she poses no threat to society.

"The primary criterion for imposing the death penalty in Texas is that the defendant is deemed to be a 'future danger' to society," said Mr Richard Dieter of the Death Penalty Information Centre.

"It is hard to see how this 60-year-old woman, confined to a wheelchair, would be a threat to society if she remained in prison for the rest of her life.

"She has exhibited multiple signs of mental illness, which may have played a role in her crime. Texas has many far more dangerous criminals serving time in prison. Executing Suzanne Basso appears both unnecessary and unjustified."

Basso, whose lawyers have also argued she is mentally disabled, will become only the 14th woman to be executed, and the fifth in Texas, among the 1,366 executions carried out since capital punishment was reinstated in the United States in 1976.