Moody's Investors Service is downgrading the senior unsecured ratings of BP PLC, the latest in a string of hits to the beleaguered oil company.

The downgrade comes three days after scientists gave a new estimate of 2.52 million gallons of crude a day leaking into the Gulf of Mexico from a well that BP was drilling.

Moody's cited the worsening impact of the oil spill and says the catastrophe will hurt its finances for years to come.

Moody's on Friday cut all of the long-term debt securities by three notches to "A2" from "Aa2." It's Moody's second downgrade for BP this month.

Moody's is reviewing the ratings and could downgrade them again. Other ratings agencies are also considering more downgrades.

Meanwhile, Standard & Poor's cut BP credit rating on Thursday and said it may cut it again, saying the oil giant's estimated liabilities to the Gulf of Mexico oil spill have increased.

S&P cut BP two notches to "A," the sixth-highest investment grade, from "AA-minus." Further downgrades are possible if costs BP faces from the spill grow, S&P said.

"BP is now subject to intense political pressure in the U.S., its largest market," S&P said. "We see these factors as fundamental issues differentiating BP from its peers."