Image caption San Quentin is one of the most famous death row sites in the US

Voters in California are to be asked whether they want to abolish the state's death penalty law.

The measure will appear on November's ballot after more than 500,000 people signed up to back the proposal.

The measure would see death row inmates have their sentences commuted to life. Just 13 people have been executed since the law was re-introduced in 1978.

Backers say abolition could save California $100m (£62m) per year, but opponents say justice would be harmed.

"Our system is broken, expensive and it always will carry the grave risk of a mistake," said Jeanne Woodford, a former warden of San Quentin Prison, home to the largest death row unit in the US.

Ms Woodford is now an anti-death penalty advocate and is named as the official proposer of the measure, which is backed by the American Civil Liberties Union (ACLU).

Ms Woodford and other supporters say the cash savings would be achieved by taking prisoners off death row and by cutting down on fees for lawyers arguing death penalty cases. The money could be better spent investigating unsolved crimes, backers of the measure say.

Under the terms of the measure those sentenced to life in prison for murder would in future have to take up jobs while incarcerated.

'Political points'

With the state of California wracked by long-standing budget issues, there is wide acceptance that the death penalty system needs reform.

Data from the Death Penalty Information Center shows that at the start of the year the state had 723 inmates on death row. The US as a whole had 3,189.

But no inmate has been put to death in California since 2006, and a respected study in 2009 noted that the state was spending some $184m each year to keep death row and the death penalty infrastructure up and running.

Opponents of the measure argue that the principle of the death penalty is valid and should remain, but say the constant and costly appeals and legal fees are inflating the costs.

"On behalf of crime victims and their loved ones who have suffered at the hands of California's most violent criminals, we are disappointed that the ACLU and their allies would seek to score political points in their continued efforts to override the will of the people and repeal the death penalty," former Sacramento prosecutor McGregor Scott told the Associated Press.

The death penalty measure is the fifth to qualify for November's ballot, California's secretary of state said on Monday.

Other measures deal with water costs, political contributions, car insurance and local legislative boundaries.