A top water official moved too slowly on a 1989 Las Vegas request for certain water rights, the Nevada Supreme Court ruled Thursday -- a finding that could delay or even kill a $3.5 billion proposal to pipe water 300 miles from Snake Valley to Sin City.

The ruling prompted Utah officials to stand down on a pending Snake Valley water-sharing agreement with neighboring Nevada. A recent Salt Lake Tribune poll of Utah voters shows across-the-board opposition to that plan.

"Based on the additional requirements imposed by the Nevada Supreme Court," Gov. Gary Herbert said, "an agreement, at this time, is premature."

The unanimous Supreme Court decision said Nevada state Engineer Tracy Taylor "violated his statutory duty" when he failed to make a decision by 1991 on 34 applications made by the Southern Nevada Water Authority for rights to water in aquifers under three Nevada valleys. Government scientists and other geology experts say those aquifers are connected with Snake Valley.

The decision focused narrowly on Taylor's timing on the water-rights applications for hundreds of wells, and points out that, in 1989, Nevada law required such requests to be decided no later than a year after the end of a formal protest period, in this case 1991.

In 2003, the Nevada Legislature amended the law to let the state engineer postpone action on water-rights applications for municipal use, but did not make that change retroactive.

Therefore, the court said, the Vegas applications weren't active past 1991.

The ruling stems from a "due process" appeal of Taylor's 2006 decision to deny petitions from at least 54 individuals and groups representing thousands of people who wanted Nevada to reopen the water-rights application protest period.

The 54 plaintiffs wanted to comment on how the Vegas pipeline drawdowns could affect landowners in both states, Utah air quality, animal and plant species and the frail hydrology of the entire Great Salt Lake Basin, matters not understood nor addressed during the original protest period.

In 2008, while that appeal was before the court, Taylor approved Southern Nevada Water Authority water-rights applications in the Cave, Delamar and Dry Lake valleys of central Nevada.

But a Nevada judge in October struck down that action, saying Taylor hadn't provided sufficient scientific proof for his estimates of the three valleys' available underground water.

The separate due-process matter now must go back to the lower court, which has two choices: Make the southern Nevada utility reapply for water rights or reopen the protest period.

Either choice would mean Utah counties, environmental groups, west desert ranchers, wildlife advocates, scientists and residents, heretofore shut out, could have a say in the proceedings.

"I'm absolutely elated," said Cecil Garland, a rancher in Callao, Utah, and staunch opponent of the pipeline and the proposed two-state water deal.

The ruling "is a dramatic change. It gives us reason to hope," Garland said. "The [Nevada] state engineer has had a kind of hegemony over the whole process. Against our will, against our protestations, we had become disenfranchised citizens. ... In my 84 years on this Earth, I have never seen politicians bow their neck and go against public opinion like this."

Salt Lake County Mayor Peter Corroon, who has made opposition to the Snake Valley accord part of his gubernatorial campaign, welcomed the ruling.

"This confirms that now isn't the right time to sign the agreement," Corroon said. "In a state where water is at an extreme premium, we need to protect it as much as possible."

Great Basin Water Network coordinator Rose Strickland called the ruling "a home run for the public."

"If we follow the law and the science," she said, "there will be no misguided pipeline threatening the environment and economies of rural Nevada and Utah."

Bob Conrad, spokesman for Taylor's office, declined to comment, saying Nevada officials still were studying the ruling.

"We now have additional opportunities to continue to look at the issue," Herbert said, "and ensure that Utah's interests are protected well into the future."