SAN FRANCISCO >> A voter-approved measure to speed up executions in California was upheld today by the state Supreme Court, but justices rejected arguments that a provision setting a five-year limit on appeals was mandatory, raising doubts that the law will succeed in accelerating death sentences.

“What happened here is that the Supreme Court of California has kicked the can down the road,” said Franklin Zimring, an expert in capital punishment at the University of California, Berkeley School of Law.

The highly anticipated ruling concerned Proposition 66, a push to “mend not end” capital punishment in California.

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.

Nearly 400 death penalty appeals are pending.

In its ruling, the state Supreme Court unanimously upheld the mandate in Proposition 66 that lower-level courts hear some appeals and reject those that are not filed on time.

It also upheld provisions in the measure calling on courts to expedite their review of capital cases, appoint attorneys as soon as possible, grant extensions only for extraordinary reasons and limit the possibility of successive appeals.

But with two of the seven justices dissenting, the state Supreme Court said a five-year deadline on appeals by condemned inmates in the measure was advisory, not mandatory — a point that supporters of the measure had conceded during oral arguments.

“The five-year period may be understood to express the voters’ view of what would ordinarily constitute ‘a reasonable time’ for completing review proceedings,” Associate Justice Carol Corrigan wrote for the majority. “However, courts must make individualized decisions based on the circumstances of each case. They retain discretion over the time required to resolve particular matters, and over the management of their dockets.”

With the timeline provision limited, it was not clear whether the measure would succeed in accelerating death sentences.

In her opinion, Corrigan acknowledged that it remained to be seen “how effective the procedures enacted by Proposition 66 will be in expediting the capital posttrial review process.”

Robert Weisberg, a criminal justice expert at Stanford Law School, said the court’s interpretation of the five-year deadline as aspirational rather than mandatory may limit the measure’s effect.

“Various people in the judicial system might say, ‘I feel an obligation to move these cases faster,’” he said.

But without a mandate, Weisberg said, others might view the deadline as “meaningless, and therefore nothing may change.”

Kent Scheidegger, an attorney who argued in support of Proposition 66 before the high court, said the five-year deadline was a minor part of the measure.

“Proposition 66 will go into effect almost entirely as written,” Scheidegger said. “Californians finally have a chance to see justice carried out in the very worst murder cases.”

Under the measure, more lawyers would have to take death penalty appeals, and they would be assigned almost immediately after sentencing. It would shift one type of appeal focused on newly discovered evidence or alleging misconduct by jurors or prosecutors to trial court judges.

Proposition 66 would also end the requirement that prison officials receive approval from state regulators for their new lethal injection plan. Regulators rejected an initial version of the plan in December.

Opponents of the measure also claimed victory.

Christina Von der Ahe Rayburn, an attorney who argued against the proposition, said the ruling had rendered the deadlines in Proposition 66 “toothless,” allowing courts to continue to perform their “critical role in carefully reviewing the appeals of the state’s death row inmates, in order to avoid the execution of an innocent person.”

“We are gratified that the California Supreme Court recognized the unconstitutionality of Proposition 66’s attempt to impose unrealistic deadlines on the courts’ consideration of capital appeals,” she said in a statement.

Death penalty opponents agreed with Proposition 66 backers that the current system was broken, but they argued that the measure would lead to the appointment of incompetent attorneys and overwhelm courts. The result: Insufficient review that could send innocent people to their deaths.

The measure — designed by prosecutors and approved by 51 percent of voters — defeated a competing initiative on the November ballot that would have abolished the death penalty.

It now takes up to five years for death row inmates to get an attorney, and it can take upward of 25 years to exhaust appeals.

Arguments before a divided California Supreme Court in June focused on whether the measure’s five-year deadline to hear appeals was realistic and enforceable. Supporters of the measure surprised observers when they conceded the time limit was not mandatory but more of a guideline.

In a dissenting opinion, Associate Justice Mariano-Florentino Cuellar said Proposition 66 clearly contained a mandatory five-year deadline for courts to complete appeals — a requirement he said was illegal. He accused the majority of misconstruing the deadline as merely an aspirational goal and validating a “bait-and-switch” on voters by the measure’s supporters.