The U.S. Supreme Court has given death-row inmates another tool to fight their sentences by allowing them to argue the most widely used method of execution in the United States is too cruel to be constitutional.

The court unanimously agreed on Monday to allow prisoners facing death by lethal injection to mount last-minute challenges to their execution by claiming the chemicals involved cause too much pain. 

Lawyers for Clarence Hill, a 47-year-old Florida prisoner sentenced to death for killing a police officer, had argued that immense pain in a prisoner's dying moments caused by the drugs would violate the U.S. Constitution's ban on "cruel and unusual punishment."

The Supreme Court has not ruled on the merits of that argument but legal experts say the decision to admit the Hill case means many other last-minute challenges to death sentences can be expected.

Lethal injection used since 1982

When lethal injection was first used in Texas in 1982, it was introduced as the most humane method of execution. It is the main form of execution used by 37 of the 38 American states that have the death penalty. Nebraska uses the electric chair.

Three chemicals are generally used in the procedure — a powerful anaesthetic, a muscle relaxant and a lethal dose of potassium chloride, which disrupts messages between the heart and the brain. 

A study published in April 2005 by the British medical journal Lancet found that up to 43 per cent of inmates might still be conscious after receiving the anaesthetic, which would leave them able to feel great pain from the potassium chloride.

Twelve American states have banned potassium chloride injection as too painful for use in the humane killing of pets.

Ruling to have impact, experts say

Deborah Denno, a law professor at Fordham University in New York City, says the Supreme Court ruling will shake up certainties about lethal injection, especially among prison officials who administer the punishment.

"I think they're going to be under much more scrutiny about what they're injecting and it really pushes them to look at other kinds of drugs — or to make sure that their executioners are going to be trained well to do the injection."

In California — which along with Florida and Texas accounts for nearly half of all American death sentences —  an execution was halted earlier this year when the only two medical doctors involved in the procedure withdrew their services. Both were anaesthesiologists. 

The American Medical Association has advised its members not to take part in executions but some doctors have disregarded that.  Most have to work anonymously and are often paid in cash for their services, to avoid public exposure and professional censure.

There are more than 3,100 prisoners awaiting execution in American jails.  The number has gone down in recent years along with a decrease in violent crime.