The bank, which is one of Germany's largest real estate financers, last week sought to reassure investors that it has enough funds to cope with a property slump that has cast a shadow over numerous lenders.

Ratings agency S&P cut PBB's rating to BBB-/A-3 from BBB/A-2 late on Wednesday with a negative outlook, sending the bank's shares down 7.5% in Frankfurt on Thursday morning.

"The negative outlook reflects our view that weak (commercial real estate) markets, notably in the U.S., could increase the cost of risk in PBB's highly concentrated loan portfolio," S&P said.

PBB declined to comment.

(Reporting by Tom Sims; Editing by Diane Craft, Kirsti Knolle and David Goodman)