The US Supreme Court has overturned death sentences against dozens of convicted killers, ruling that juries and not judges must make the decisions.

This case presents a question of who decides: judge or jury. The context is capital murder, the issue, life or death

Justice Ruth Bader Ginsburg

The 7-2 ruling affects the way death sentences are imposed in Arizona and at least four other states, with implications for the fate of more than 150 killers - and possibly as many as 800.

Their death sentences could now be commuted to life imprisonment.

About 3,700 people currently await execution across the United States, although most of them will still be put to death.

The court ruled that a sentence imposed by a judge violates a defendant's constitutional right to a trial by jury.

The states affected have previously allowed juries only to determine the guilt or innocence of defendants, while judges decided whether there were aggravating factors meriting the death penalty.

States affected

Arizona Idaho Montana Colorado Nebraska Alabama Delaware Indiana Florida

Opponents of the death penalty are delighted, particularly as this is the second Supreme Court verdict curbing the use of capital punishment in less than a week.

On Thursday, the court ruled that executing mentally disabled killers was unconstitutional because it was "cruel and unusual" punishment.

Restricting move

The two decisions do not mean that the Supreme Court justices are moving towards outlawing the death penalty, says the BBC's Washington correspondent Justin Webb.

But they are showing a willingness to curb its use where legal or social arguments are compelling, he says.

Monday's Supreme Court ruling will immediately apply in that state and in Idaho and Montana, where a single judge decides the sentence.

Ring was convicted of murdering a security guard during a robbery

It will also apply immediately in Colorado and Nebraska, where a panel of judges makes the sentencing decision.

In four other states - Alabama, Delaware, Florida and Indiana - where juries make sentencing recommendations, but judges have the final decision, death-row inmates may also challenge their sentences.

Ring v Arizona

The Supreme Court was considering the case of an Arizona inmate, which rested on the fact that nine of the 38 states which retain the death penalty leave sentencing up to judges, rather than jurors.

The ruling is a victory for Timothy Stuart Ring, who was sentenced to death by an Arizona judge for the 1994 killing of a security guard in Phoenix.

After the verdict, the jury was discharged.

The judge at a separate hearing said Ring deserved the death penalty after finding two aggravating circumstances - committing the murder for financial gain and carrying it out in an especially heinous way.

Ring's lawyers argued the Arizona death penalty law was put in doubt by a Supreme Court ruling two years ago that overturned a hate crime sentence imposed by a judge on a New Jersey man.

Three Florida death row inmates - Amos King, Linroy Bottoson and Robert Trease - were given a stay of execution pending the outcome of Ring v Arizona.