S&P Global Ratings on March 27 downgraded the issuer credit ratings on DPL Inc. and its subsidiary Dayton Power and Light Co. to BB- from BB, with a negative outlook.

The downgrade was driven by DP&L's decision to retire the 2,308-MW J.M. Stuart and 600-MW Killen Station coal-fired power plants in Ohio by the end of June.

The AES Corp. subsidiary signaled its intention to retire the Stuart and Killen plants as part of an agreement in principle reached earlier this year with the Sierra Club. The agreement is tied to a larger settlement with intervenors in the Ohio company's electric security plan.

"The downgrade on DPL and DP&L reflects our base-case scenario that over the next few months the Public Utilities Commission of Ohio will most likely approve the distribution modernization rider in line with the settlement proposal," the rating agency said in a report. "The negative outlook reflects our expectations for persistently weak financial measures for consolidated DPL that are now highly dependent on the long-term sustainability of the DMR. The company also faces significant refinancing risks for its approximate $1.7 billion of outstanding debt, maturing 2019-2022."

S&P also lowered its rating on DPL's senior unsecured debt to B+ from BB, and affirmed its BBB- rating on DP&L's senior secured debt.