The execution of Kevin Cooper, who was convicted of the 1983 murder of a family of four in California, was stayed by the US supreme court to allow DNA testing of evidence found at the crime scene.

The reprieve could hardly have come any later for Cooper. The 46-year-old was sitting with a spiritual adviser in a room next to the execution chamber when news of the stay came through. His arm had already been prepared for the lethal injection.

The news was announced to 300 supporters of Cooper gathered outside the gates of San Quentin state prison. "Thank you for letting the governor know he was wrong," an anti-death penalty activist, Mike Farrell, told the crowd.

But it is the presence of Cooper's more high-profile backers that has raised the profile of his case.

The actors Sean Penn, Richard Dreyfuss and Denzel Washington have all drawn attention to Cooper's case, and the Rev Jesse Jackson has campaigned vigorously on the issue.

"This is part of a struggle across a nation to remove a system with flaws," Mr Jackson told supporters at the prison. Earlier in the day, he had met Cooper.

But others have questioned the strategy adopted by Cooper's defence team. "Cooper is different from those who've been executed before him in this state," Lieut Vernell Crittenden, who supervises San Quentin's cells and has met Cooper several times in the last week, told the Los Angeles Times.

"Others use legal counsel to sway the judicial system. He has actively solicited public support to do it."

Should Cooper be executed, it would be the 11th execution since California reinstituted the death penalty in 1978, and the first since Governor Schwarzenegger came to office.

The state has the highest prison population in the US, and while Mr Schwarzenegger has suggested that he will investigate corruption in the prison system, it is thought that the film star-turned-Republican politician will be hardline on the death penalty.

He released a statement yesterday extending condolences to the family of the victims: "To the survivor and families of the victims in this case, I share your frustration that closure to this chapter of your lives has been delayed."

The mother of one of the victims, 11-year-old Christopher Hughes, said Cooper "knows now what it feels like to come within four hours of being executed. Now he can sit in his cell and think about it for another 40 days".

The boy's father said: "This is like a prize fight. We've been knocked down but we're not out and we'll win eventually."

Cooper was convicted in 1985 of the murders of Douglas and Peggy Ryen, both 41, their 10-year-old daughter, Jessica, and Christopher Hughes. The Ryens' son, Joshua, then 8, survived a slit throat.

When the murders were committed, Cooper was on the run after escaping from prison, where he had been serving a four year sentence for burglary.

Cooper's defence argues that the killings were committed by three murderers, and says that forensic evidence was planted by police. The stay of execution will allow authorities to run tests on a bloody T-shirt found at the scene of the crime and a hair found in the hand of one of the victims.

Although the T-shirt has Cooper's blood on it, his defence argues it was planted by police. Testing will show whether a chemical used by police to preserve blood evidence is on the shirt. If it is, argues Cooper's legal team, it will show that the blood was placed on the T-shirt by the police after the murders.

But prosecutors are convinced they have the right man. "We are confident the results of future tests will not cast doubt about Cooper's guilt," said a spokeswoman for the attorney general.

The supreme court judges behind the 9-2 decision to order a stay of execution were more outspoken.