SAN FRANCISCO (Reuters) - The California Supreme Court on Thursday said a ballot initiative that aimed to speed up death penalty cases could not impose mandatory deadlines on appeals, but it also rejected broader constitutional challenges to the measure.

Voters narrowly passed Proposition 66 last year, while a companion measure that sought to abolish the death penalty failed.

It aimed to speed up the appeals process for death penalty cases by imposing a five-year limit for appeals and restricting petitions known as habeas review, which lawyers use to bring in arguments that were not heard in the original trial.

With the exception of the five-year timeline, the rest of the measure was upheld, and will go into effect next month.

“Prop. 66 preserves the death penalty for the most heinous criminals by enacting critically needed reforms to the system,” said Michele Hanisee, a supporter and president of the Association of Deputy District Attorneys.

The death penalty has come under increasing scrutiny in the United States in recent years. Critics contend it is applied unevenly, and that methods of lethal injection violate constitutional protections against cruel punishment.

In California, which has not conducted an execution since 2006, inmates on death row can await execution for decades while their appeals make their way through the court system. Backers of Proposition 66 argued that crime victims deserved “timely justice” and that bureaucratic delays should be curtailed.

It instituted a series of new deadlines and raised the standard for filing certain kinds of legal appeals.

The measure was challenged in court by plaintiffs including a former state attorney general, who argued it would cause “confusion and upheaval” in the courts and suppress legitimate challenges to death penalty convictions.

In the ruling on Thursday, the California Supreme Court in a 5-2 split said the deadlines in Prop 66 should be treated as directive, not mandatory, to avoid “serious” separation of powers issues. They said the rest of the initiative was not on its face unconstitutional.

But death penalty attorney Robert Sanger, who filed an amicus brief supporting the challenge to the measure, said the court left open a key avenue for challenging the initiative. While it may not be unconstitutional on its face, he said, attorneys can still argue that the measure was applied in a way that violates a defendant’s constitutional rights.

“There are going to be lawyers trying to find ways of saving lives,” Sanger said. “I will raise every issue I can.”