Moody’s Investors Service has cut the ratings it has assigned to UnitedHealth Group Inc. 1 notch.

A review team hired by the UnitedHealth board announced Sunday that it had found evidence that the company had backdated options and had suffered from weak internal controls.

Moody’s, New York, has responded by cutting the rating it has assigned to long-term UnitedHealth debt to A3, from A2.

Moody’s also cut the insurance financial strength rating of United HealthCare Insurance Company, UnitedHealth’s main operating subsidiary, to A1, from Aa3.

Standard & Poor’s Ratings Services, New York, announced Monday that it was affirming the A counterparty credit rating it has assigned to UnitedHealth.

The Chicago office of Fitch Ratings said the A plus long-term issuer default rating would stay on “rating watch negative.”

At Moody’s, “the outlook on the ratings remains negative, reflecting the uncertainty regarding the ultimate financial, operational, and reputational impact to the company,” Moody’s says in a discussion of the rating cuts.

Although UnitedHealth has announced executive departures and says it is taking a number of actions to address the review team’s concerns, changing the culture of the company’s board may take some time, Moody’s says.

“While Moody’s believes that accounting restatements and tax implications are not expected to materially impact the company, potential penalties, fines, and litigation may be significant,” Moody’s says. “In addition, delays in filing the company’s 10-Q’s for the second and likely the third quarters have left the company vulnerable to action by bondholders. UnitedHealth could also sustain damage to its reputation, which may result in non-renewal of business by employer groups or in providers leaving UnitedHealth’s network, although Moody’s considers a significant loss of membership or providers unlikely.”

But UnitedHealth has made smart acquisitions and done a good job of increasing enrollment at existing operations, Moody’s says.

The company also has an excellent customer and product mix and excellent cash flow, Moody’s says.

Moody’s says it might lower UnitedHealth’s ratings further if the company faces regulatory, tax or litigation costs greater than $2 billion, or if there are findings of criminal wrongdoing by the U.S. Department of Justice.