A man with an IQ of 70 has been executed in the US state of Georgia following years of legal battles and arguments he should have been saved by the Constitution.

Warren Hill, a mentally disabled prisoner who has been on death row for the past 24 years, was executed by lethal injection for murdering a fellow inmate in 1990. He was already serving a life sentence for killing his girlfriend in 1986.

The 54-year-old, who has the mental age of a child, had three last-minute stays of executions before he was finally killed by the state of Georgia.

His lawyers had long argued Hill is intellectually disabled and therefore cannot be executed.

The US supreme court ruled against the execution of prisoners with mental disabilities in 2002. However, the court left each state with the authority to determine what constitutes mental disability.

Seven medical experts who examined Hill, including three who were appointed by the state, concluded that he was mentally impaired by a "preponderance of the evidence".

However, Georgia is the only state which insists any prisoner due to be executed must prove "beyond a reasonable doubt" that they are that they are intellectually impaired - a requirement that is almost impossible to achieve.

Brian Kammer, a lawyer for Hill, said his client's death amounts to a "grotesque miscarriage of justice".

He added: "Georgia has been allowed to execute an unquestionably intellectually disabled man, in direct contravention of the supreme court's clear precedent prohibiting such cruelty.

"The memory of Mr Hill's illegal execution will live on as a moral stain on the people of this state and on the courts that allowed this to happen."

Hill was sentenced to serve life in prison for shooting his 18-year-old girlfriend Myra Wright in 1986. While serving that sentence, he beat a fellow inmate, Joseph Handspike, to death using a nail-studded board. He was convicted of a second murder and a jury sentenced him to death in 1991.