PG&E Corp. stock tanked more than 25% Monday as investors grappled with an unfavorable court ruling that could make the company’s bankruptcy case much more expensive.

Shares closed at $10.67, a $3.61 drop from Friday. The plunge came after U.S. Bankruptcy Judge Dennis Montali said Friday afternoon that a state court trial can determine whether PG&E is responsible for the 2017 Tubbs Fire, the second most destructive wildfire in California history.

The state previously said the fire was caused by privately owned electric equipment, not PG&E’s lines. If attorneys for several Tubbs Fire victims convince a jury to rule in favor of their clients, it will make the company’s bankruptcy billions of dollars costlier.

“The operative word here is uncertainty,” said Paul Patterson, an analyst for the Glenrock Associates utility research firm in New York, of Montali’s decision. “It’s not a positive event, but what it actually means financially — I don’t know if that can be ascertained at this point. It’s clearly what the company wanted to avoid.”

Mike Danko, an attorney for Tubbs Fire victims suing PG&E, said the company’s liability for the Tubbs Fire alone could reach $18 billion by some estimates. PG&E said when it filed for bankruptcy protection in January that its liabilities from all wildfires could exceed $30 billion.

Victims’ lawyers are hoping to fast-track a jury trial in San Francisco Superior Court and think they can secure a verdict by January or early February. They will argue not only that PG&E is responsible for causing the Tubbs Fire, but also that the company was negligent.

If a jury agrees, it could create more problems for PG&E.

“That would be a direct hit to shareholders,” said Travis Miller, a utilities analyst for Morningstar Research Services, of a potential negligence finding. “I’m presuming it would have an impact on the dollar amount of the court decision. It would also presumably have an impact on how regulators treat the company in terms of fines and other rulings that they could put in place.”

PG&E was fined a record $1.6 billion by the state utilities commission in connection to the 2010 San Bruno pipeline explosion, Miller noted.

“We do not comment on market speculation,” said PG&E spokeswoman Lynsey Paulo in an email.

Cal Fire announced the results of its investigation into the Tubbs Fire in January, more than 15 months after the blaze ignited near Calistoga and burned into Santa Rosa, killing about two dozen people and destroying more than 5,600 buildings. Cal Fire concluded the cause was a private electrical system not controlled by PG&E, which had been blamed for other wildfires that ignited around Wine Country during the same October 2017 firestorm.

While PG&E has argued it was cleared of responsibility for the Tubbs Fire, victims’ lawyers never agreed with the state’s conclusion. They asked Montali to let them try their case in state court, where they are prepared to argue that Cal Fire was wrong.

Attorneys believe the report’s methodology was flawed, that evidence from PG&E equipment near the origin site suggests the company was responsible, and that the property where the state says the fire started did not have power at the time the Tubbs Fire began.

PG&E does not have to let the case go to trial. The company could always settle.

“I would expect, once cards get flipped over, that PG&E will say, ‘OK, let’s talk,’” said Danko, the fire victims’ lawyer. “The best way of forcing the issue of reasonable negotiations is to have a Tubbs Fire trial looming.”

If the trial does occur, Danko said PG&E would not be able to pass costs related to negligence or punitive damages along to customers, worsening the potential financial consequences.

But Michael Wara, director of Stanford University’s energy policy program, doubted PG&E could settle with Tubbs Fire victims because the sides are too far apart.

“The Tubbs Fire victims think PG&E started the fire. They don’t buy the Cal Fire report at all,” Wara said. “The plaintiffs are not inclined to settle in terms that are anywhere close to what PG&E or the bondholders have proposed.”

More Information $14.28 Closing price on Friday $10.67 Closing price on Monday $49.42 52-week high

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State regulators already blocked PG&E from a new process they established for utilities to raise rates in connection with 2017 wildfire costs. Following the direction of a new wildfire law, the utilities commission created a financial “stress test” to determine how much of a burden companies can bear before looking to raise rates due to a 2017 disaster.

The commission prevented PG&E from using the stress test because of its bankruptcy. PG&E has asked for a rehearing.

Separately, Montali, the bankruptcy judge, will have to estimate the company’s liability for all wildfires, including the historic 2018 Camp Fire that was started by PG&E electrical equipment. He wrote Friday that the estimation process can advance while the Tubbs Fire trial proceeds.

Danko said the result of the Tubbs Fire trial would become a “data point” for the judge to consider while estimating PG&E’s total fire costs.

PG&E is under pressure to resolve its bankruptcy case by June 30 so it can use a new fund the state authorized to protect electric companies from future wildfire costs. PG&E’s exit from bankruptcy, which will create a way to pay claims from past fires, must have a neutral impact on customer rates in order to comply with the law.

J.D. Morris is a San Francisco Chronicle staff writer. Email: jd.morris@sfchronicle.com Twitter:@thejdmorris