Turkey’s political instability and the global credit squeeze combined on Thursday to undermine perceptions of the country’s creditworthiness, capping a week of bad news for investors and the Ankara government.

In a surprise announcement, Standard & Poor’s, the credit rating agency, cut its outlook for Turkey’s creditworthiness from stable to negative, in effect ending the prospect that the country’s rating would be raised this year. Turkey currently has a S&P rating of BB-, three notches below investment grade, the benchmark that allows governments to borrow at cheap rates.