PG&E Corp.  (PCG - Get Report) shares extended declines Tuesday following a move by Standard & Poor's to cut the California utilities credit rating into junk status as it grapples with billions in costs associated with last year's Camp Fire disaster.

S&P Global Ratings lowered PG&E's credit rating to single-B, two notches below the investment grade threshold, affecting around $18 billion in outstanding bonds and sending the group's shares more than 22% lower on Wall Street.

Last November, PG&E warned investors that it faced "significant" liability, over and above its insurance capacity, if its equipment was found to have started the deadly Camp Fire that started in the city of Paradise and spread through much of Northern California, killing at least 86 people and causing billions in economic and environmental damage. 

"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. "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."

PG&E shares fell 10% on Tuesday to $17.05. PG&E shares have lost more than two-thirds of their market value since the Camp Fire was first reported on November 8.

Reuters reported last week that the San Francisco-based utility is exploring bankruptcy protection in the face of what CNBC reported could be $30 billion in penalties linked to the Camp Fire. 

Last week, PG&E's board of directors said it was "actively assessing PG&E's operations, finances, management, structure, and governance -- and remains focused on improving safety and operational effectiveness," in addition to the prior actions it had taken to "confront the growing wildfire threat."

The California Public Utilities Commission said last month it was investigating PG&E's role in falsifying data related to the safety of its natural gas pipelines.

"Utility falsification of safety related records is a serious violation of law and diminishes our trust in the utility's reports on their progress," the Commission said. "These findings are another example of why we are investigating PG&E's safety culture."