Troy Davis says he did not kill a policeman in Georgia in 1989

The US Supreme Court has ordered that a death row inmate should receive a new hearing to see if evidence not heard at his trial proves his innocence.

Troy Davis was convicted in 1991 of killing a policeman in the southern state of Georgia, but key witnesses have recanted their testimony.

In September the Supreme Court granted him a stay of execution hours before he was due to die from lethal injection.

His supporters include Pope Benedict XVI and US ex-President Jimmy Carter.

Lawyers for Davis said in their appeal that seven of nine prosecution witnesses had retracted their trial testimony.

The lawyers also said several new witnesses had identified or implicated a different individual as the person who killed police officer Mark MacPhail in a car park in 1989.

Attorneys for the state of Georgia had argued that the appeal should be rejected.

They said each court that had reviewed claims by Davis had said he had failed to prove his innocence.

Conservative justices Antonin Scalia and Clarence Thomas dissented from the Supreme Court decision.

Justice Scalia said the Supreme Court was sending the federal judge in Georgia on a "fool's errand"

But Justice John Paul Stevens, supported by Justices Ruth Bader Ginsburg and Stephen Breyer, said Justice Scalia was wrong.

"The substantial risk of putting an innocent man to death clearly provides an adequate justification for holding an evidentiary hearing," wrote Justice Stevens.