S&P affirms Portugal BB long-term rating; outlook negative

"The ratings affirmation reflects our view of the significant structural reforms the Portuguese government has undertaken in the past 12 months amid rapidly narrowing current account deficits, mostly reflecting strong export performance," S&P said in a statement.

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"The outlook is negative, mainly reflecting our view of downside risks arising from the euro zone debt crisis and, in particular, risks associated with Portugal's close trade and financial links with Spain."

But the country's ratings "could stabilize at the current level if the government's budgetary performance and structural reform measures continue as envisaged in the program," the statement read, which could boost economic fundamentals.

In June Portugal passed the fourth quarterly review of its 78-billion-euro bailout program by the European Union and International Monetary Fund, vowing to stick to the pact's goals.

Moody's Investors Service rates Portugal Ba3, and Fitch rates the country BB-plus. Both ratings carry a negative outlook.