The lowering of Liberty Mutual Group’s financial strength rating from A to A-/stable yesterday by Standard & Poor’s came as a “surprise” to the Boston-based insurance company.

The lowered rating came after Liberty Mutual completed the acquisition of Safeco Corp. this past Monday.

The announcement was in contradiction of prior announcements, the company said. When Liberty and Safeco announced the transaction in April, Standard & Poor’s placed Liberty’s ratings on CreditWatch with negative implications. In that announcement, Standard & Poor’s stated that they expected to affirm Liberty’s ratings if its capitalization was not materially below what is required for the current rating, and were satisfied with Liberty’s integration efforts of Safeco.

Liberty said its reported pre-tax operating income of nearly $900 million in the first six months of 2008 was approximately 3 percent higher than the same period in 2007. In addition, Liberty’s pro forma capital adequacy using Standard & Poor’s published criteria indicates Liberty’s capital is adequate to maintain its ‘A’ rating.

“The turbulence and uncertainty in today’s financial and credit markets has focused a negative spotlight on Standard & Poor’s process,” the company said in a statement. “As a result, one would have hoped that now more than ever Standard & Poor’s would have recognized its fiduciary obligation to act in a consistent, coherent and accurate manner. Unfortunately, they have not.”