Warren Buffett’s Berkshire Hathaway Inc., owner of the largest stake in Moody’s Corp., may have lost almost $300 million on the holding this week as shares of the credit-ratings company plunged.

Moody’s, the second-biggest provider of credit ratings, dropped 19% through yesterday after larger rival Standard & Poor’s said on Feb. 4 that it could face a U.S. lawsuit over inflated mortgage-bond ratings.