By Jeffrey Sparshott Of DOW JONES NEWSWIRES LONDON (Dow Jones)--Ratings agency Moody's Investors Service Friday downgraded BP PLC's (BP) credit rating three notches as costs and exposure to litigation from the Gulf of Mexico oil spill continue to mount. It is the third major whack at BP's credit rating in recent days. "Moody's updated assessment is that the spill will have a sustained negative impact on the group's free cash flow generation and overall financial profile for a number of years," the ratings agency said. Moody's lowered BP's senior unsecured ratings to A2 from Aa2. Standard & Poor's Ratings Services Thursday lowered its rating on BP by two notches, its second downgrade on the oil giant this month, while Fitch Ratings downgraded BP long-term debt six notches to just above junk level Tuesday. Moody's said its second downgrade of BP this month reflects a substantial upward revision of the estimated size of the leak, failure to bring the well under control, and the mounting costs and claims for damages. All BP ratings remain on review for possible further downgrade, Moody's said. -By Jeffrey Sparshott, Dow Jones Newswires; +44 (0)207 842 9347; jeffrey.sparshott@dowjones.com (END) Dow Jones Newswires June 18, 2010 08:00 ET (12:00 GMT)