MILPITAS — The cost of Gov. Jerry Brown’s controversial plan to build two massive tunnels to move water from north to south increased more than a billion dollars to $24.7 billion on Wednesday, but officials insisted it will be a bargain for water users around the state.

Critics of the project have attacked the administration for its delay in releasing figures outlining the benefits of the project. But Wednesday’s release of the final chapters of the Bay Delta Conservation Plan for the first time hung a price tag on the benefits — at least $5 billion over the next 50 years.

“The current water supply is vulnerable, and the cost of its failure would be enormous,” said John Laird, the state’s secretary of natural resources. “As public officials, we are duty-bound to address these threats. (This plan) provides the most comprehensive, well-conceived approach to ensuring a reliable water supply to 25 million people and restoring the Delta ecosystem.”

Added Mark Cowin, director of the state Department of Water Resources: “When you consider the current trends in the Delta, I think the real question is, ‘Can we afford not to?'”

The $5 billion figure comes from projected improvements in the reliability of the water supply, better water quality and reduced seismic risk, according to David Sunding, principal of the Braddle Group and an economics professor at UC Berkeley. His firm conducted the cost analysis.

The release of the economic portion of the plan is expected to prompt another skirmish in the decades-old California water wars that have stymied politicians, blocked progress and left environmentalists and developers in a state of perpetual antagonism.

With every move the administration makes, opponents chip away with criticism and dangle the threat of lawsuits that could further delay resolution of a problem that most agree exists: a water supply system that is perennially at the edge of crisis and a fragile eco-system that needs to be shored up.

Add to the mix Brown’s hopes of going down in history as prolific as his governor father in creating lasting, visible signs of his political craftsmanship on California’s landscape.

The new battle has also evoked comparisons to Brown’s first term, when he became embroiled in a fight over the so-called Peripheral Canal, which he approved in 1981 but saw voters repeal in a heavy-handed rebuke of his first attempt at taming California’s water politics. The proposed canal, which like the current project also would have diverted water around the Delta, sparked a bitter campaign that pitted Northern California voters against Southern California voters.

About 68 percent of the new Delta plan would be covered by water users through higher rates, while about 15 percent would come from taxpayers by way of two future water bonds, including one set for 2014.

Silicon Valley business and water officials pointed out Wednesday that a sizable portion, about 40 percent, of the region’s water is drawn from the Sacramento-San Joaquin River Delta. The same is true of water districts in southern Alameda County, Santa Clara Valley Water District CEO Beau Goldie said.

“We want the plan out there and transparent so we can talk from facts,” Goldie said. “Businesses in the area significantly depend on reliable water supplies.”

Referring to recent Bay Area visits of Texas Gov. Rick Perry and Utah Gov. Gary Herbert — both intent on getting companies here to relocate to their states — Goldie said, “We don’t want to give them one more argument.”

Backers of the twin tunnels include farm and business leaders, along with labor unions and many of the state’s largest water districts, from the Metropolitan Water District in Los Angeles to the Westlands Water District in Fresno. They contend that the tunnels will provide a more reliable supply of water by reducing the need for the giant pumps near Tracy to draw as much water directly from the Delta. That pumping for years has harmed fish by crushing them, disrupting their migrations and making rivers run backward.

Opponents include environmentalists, fishing groups and a dozen Bay Area members of Congress.

Capital costs to bring Brown’s plan to fruition would total $19.9 billion — $5.4 billion of which would go toward environmental restoration, according to the new details released Wednesday by the state. About three-fourths of that cost would be in constructing the two side-by-side 40-foot-high underground tunnels to carry fresh water 35 miles from the state’s largest river, the Sacramento, under the Delta to giant pumps in Tracy. The plan also calls for $4.8 billion in operation and maintenance costs over the next half-century.

The plan was downgraded last summer by state and federal officials to three water intakes instead of five, thus taking in two-fifths less water.

That prompted University of the Pacific economist Jeffrey Michael to note that the cost of the tunnels alone has increased in little over a year from $12.8 billion to $14.5 billion “even though the water delivery capacity of the tunnels was decreased by 40 percent.”

The estimated cost increases result from inflation and having a more complete plan, creating a gravity-based flow system to move the water with longer and wider tunnels, said David Zippin of ICF International, an environmental consulting firm.

Stockton-based Restore the Delta noted that the Brown administration’s latest cost estimate for building the tunnels has more than tripled from initial estimates.

“The first estimate was $4 billion — and is now more than $14 billion,” said Barbara Barrigan-Parrilla, the group’s executive director. “This will not restore the Delta, and the cost will be shifted to taxpayers so that they can support water deliveries for a few water takers.”

The environmentalists’ main fear is that once built, the project will become a giant spigot to divert even more fresh water from the Delta to Central Valley farms and cities in the south. They say that would further harm endangered species and turn the Delta into a stagnant, saline mess, ruining the livelihoods of Delta farmers.

Staff writer Paul Rogers contributed to this story.