Supporters and opponents of a voter-approved measure to speed up executions in California are both claiming victory following a California Supreme Court ruling on its constitutionality.

The court on Thursday upheld the measure, but said its requirement that courts resolve appeals by condemned inmates within five years was advisory, not mandatory.

The highly-anticipated ruling Thursday concerned Proposition 66, a push to "mend not end" capital punishment in California. It aimed to expedite death sentences in part by setting a five-year deadline on court appeals.

Christina Von der Ahe Rayburn, an attorney for opponents of the proposition, said the Supreme Court had rendered the deadlines in Proposition 66 toothless. She said that would allow courts to continue their critical role in carefully reviewing the appeals of the state's death row inmates to avoid the execution of an innocent person.

Kent Scheidegger, an attorney for supporters of the measure, said Proposition 66 would go into effect almost entirely as written and Californians finally had a chance to see justice carried out in the very worst murder cases.

A divided state Supreme Court said the five-year deadline was advisory, not mandatory. Supporters of the measure had conceded as much during oral arguments.

The measure beat a competing initiative on the November ballot that would have abolished the death penalty.

Condemned inmates in California currently languish for decades and are more likely to die of natural causes than from lethal injection. There are nearly 750 inmates on death row and only 13 have been executed since 1978 - the last in 2006.