MISSOURI (REUTERS) - Missouri is preparing to execute convicted murderer Ernest Johnson on Tuesday (Oct 5) evening after Governor Mike Parson declined to grant the clemency sought by Pope Francis, among others, on the grounds that Johnson is intellectually disabled.

Johnson, 61, was found guilty by a jury of murdering three convenience store employees in 1994. He is due to be injected with a lethal dose of pentobarbital, a powerful barbiturate, at 6 pm local time in the state's execution chamber in the city of Bonne Terre.

His lawyers say there is overwhelming evidence of Johnson's intellectual disability. They unsuccessfully asked the US Supreme Court on Tuesday to halt the execution because executing intellectually disabled people breaches a constitutional ban on"cruel and unusual punishments."

The conservative-majority Supreme Court rarely blocks executions, and denied Johnson's petition in a brief unsigned order late on Tuesday afternoon. The Missouri Supreme Court rejected Johnson's intellectual disability claim in August.

Johnson was born with fetal alcohol syndrome disorder, has scored low on IQ tests throughout his life and has the "daily living skills" of a 4-year-old child, his lawyers said.

The Pope and former Missouri Governor Bob Holden, a Democrat who said 20 men were executed during his own tenure, have asked that Johnson's life be spared.

US House Representatives Cori Bush and Emanuel Cleaver II, both Democrats representing Missouri, also asked Governor Parson, a Republican, for clemency.

Parson denied clemency on Monday for what his office called a "brutal murder." "Mr Johnson's claim that he is not competent to be executed has been reviewed and rejected by a jury and the courts six different times, including a unanimous decision by the Missouri Supreme Court," Parson's office said in a statement.

A jury found Johnson guilty of bludgeoning to death Mary Bratcher, Mabel Scrubbs and Fred Jones using a hammer, a screw driver and a gun.