Standard & Poor's on Friday joined other credit rating agencies in downgrading BP PLC, saying the oil spill at a BP facility in the Gulf, which is costing billions of dollars to clean up, had damaged its reputation.

S&P analyst Simon Redmond said there was an "excellent" risk that the fallout from the spill would impair BP's business for a long time.

The agency downgraded BP's long-term rating to "AA-minus" from "AA" and said further downgrades were possible. Moody's and Fitch Ratings announced similar downgrades earlier this week.

BP shares fell $1.61, or 4 percent, to $37.66 in afternoon trading.

The company has lost nearly $72 billion in market value since the April 20 deepwater blowout that led to the spill.