MBIA Inc. had the BBB ratings on its municipal bond insurance unit affirmed by Standard & Poor’s today after an overhaul of the ratings company’s criteria. Its shares rose by the most since September 2009.

S&P said the business and financial risk profile of MBIA’s National Public Finance Guarantee Corp. unit was “strong” based on the updated criteria. The outlook remains “developing” because of two-year-old litigation that is challenging MBIA’s 2009 restructuring. This allowed the insurer to spin off the municipal insurance business into the new unit and separated it from a subsidiary that guaranteed soured mortgage-backed debt.

National Public Finance has been shut out of the market for writing new bond insurance while the litigation continues.

“We believe the strong and stable stream of earned premiums and investment income contribution will support National while it remains inactive, and its strong distribution and management team may position it well in the market once the litigation is settled,” S&P said in the statement.

S&P also affirmed its B rating on MBIA Insurance Corp., the old subsidiary that held onto the mortgage security guarantees. The outlook on the unit remained negative.

MBIA extended gains after the statement, climbing as much as 19.6 percent to $9.77, before falling to $9.70 as of 4 p.m. New York time.