S&P Global Ratings on Monday stripped California power utility PG&E of its investment-grade credit rating and kept it under review for a further downgrade, citing political and regulatory pressure and uncertainty as it faces massive claims stemming from deadly wildfires.

The utility, whose roughly $18 billion in bonds fell on Monday due to bankruptcy fears, has come under severe pressure since a fatal Camp fire in November compounded the company's woes. It currently faces billions of dollars in liabilities related to wildfires in 2017 and 2018.

S&P cut the rating on both PG&E and its Pacific Power & Gas Co operating utility to "B" from its previous rating of "BBB-," the lowest tier of so-called investment-grade ratings.

The ratings agency said it could further cut the company's rating over the next few months if explicit steps are not taken by authorities to improve the regulatory situation, signaling that the agency may be losing faith that lawmakers could rescue PG&E.

"We could also lower the ratings by one or more notches if management does not clearly articulate specific steps it will take to preserve credit quality over the long term," S&P said.

Earlier on Monday, PG&E shares dived more than 22 percent and its largest bond, a $3 billion note due in March 2034 with a coupon of 6.05 percent, fell to a record-low bid price of 91.5 cents on the dollar, while its yield rose to nearly 7 percent.

"We expect that negative public sentiment and the increased political pressure will challenge the regulators' willingness and ability to implement measures to protect credit quality over the near term," S&P said.

The agency also said it expects PG&E's capital access may be limited to secured debt issuance, restricting its financing options, because of the increased credit risks, and media speculation on a potential bankruptcy.

On Friday, sources told Reuters that PG&E was exploring filing for bankruptcy protection. The company was considering the move, for some or all of its businesses, as it fears a massive charge in the fourth quarter related to potential liabilities from wildfires.

The company said it was reviewing its "structural options" and assessing its operations, finances, management, structure and governance. It is searching for new directors at its holding company and its utility unit Pacific Gas and Electric Co.

In November, PG&E said it could face "significant liability" in excess of its insurance coverage if its equipment was found to have caused last year's fires in northern California.

The fires near the Northern California mountain community of Paradise swept through the town, killing at least 86 people in the deadliest and most destructive wildfire in the state's history. PG&E had reported equipment problems near the origin of the fire around the time it began.

Credit ratings for PG&E and its Pacific Gas & Electric unit were downgraded by the three main ratings agencies in mid-November.

Last month, the California Public Utilities Commission opened a proceeding to consider penalties against the company, ordering immediate action against the utility for falsifying safety documents for natural gas pipelines.