A man whose lawyers say is clinically "mentally retarded" is set to be put to death by lethal injection in Texas.

Marvin Wilson was convicted of fatally shooting a police drug informant nearly 20 years ago, with the sentence of death by lethal injection.

The US Supreme Court soon afterwards found the penalty to be in violation of the constitution’s ban on cruel and unusual punishment when imposed on the mentally retarded.

Commenting on the case, Simon Whitaker, a clinical psychologist from the UK, told Al Jazeera: "This is a miscarriage of justice, he [Wilson] would have the same reasoning powers of a five-year-old child, if a five-year-old child killed someone we would not execute them in part.

"Texas courts have judged Wilson as not mentally retarded - even though psychologists measured his IQ in the bottom one percentile, about equal to that of an average five-year-old child.

"With his diagnosis he cannot make rational decisions, there will be a clear problem for him to think things through," said Whitaker.

Texas is one of 33 states, along with the federal government, which still maintains the death penalty, which still has widespread support.

A recent poll indicated more than 70 per cent of Texans approve of capital punishment for murder.