SOFIA, Dec 1 (Reuters) - Credit ratings agency Standard & Poor’s raised its sovereign rating for Bulgaria to BBB-minus from BB-plus on Friday, citing the Balkan country’s improving external metrics, expansion of exports and rise in domestic savings.

S&P said in a statement that Bulgaria’s public financial management has been prudent, in deference to the currency board regime.

“We are therefore raising our sovereign credit ratings on Bulgaria to ‘BBB-/A-3’ from ‘BB+/B’,” the agency said.

S&P lowered the outlook on Bulgaria’s sovereign credit rating to stable from positive, adding that an upgrade could happen in the next two years if the government’s fiscal performance strengthened further beyond expectations.

“The stable outlook reflects balanced risks to the ratings on Bulgaria,” S&P said. It said it might take a positive action if further reductions in the Bulgarian banking sector’s nonperforming loans improved the country’s monetary flexibility.

Another upgrade trigger could be Bulgaria’s admission into the European Union’s exchange rate mechanism (ERM-2), commonly known as the euro’s waiting room.

S&P said Bulgaria’s relatively low income levels, weak institutional settings and adverse demographic profile constrain the ratings.

“Another credit weakness, in our view, is the Bulgarian government’s and central bank’s limited policy flexibility due to the country’s fixed exchange rate regime,” the agency said. “We recognize, however, that Bulgaria’s currency board has been an important anchor of stability for the country.”

On Thursday Bulgaria’s parliament approved a state budget bill that targets a fiscal deficit of 1.0 percent of economic output as it gears to spend more on education and wages. Bulgaria is expected to end this year with a balanced budget.

The bill assumed the economy will grow by 4.0 percent in 2017 due to strong domestic demand and exports, with growth edging down to 3.9 percent in every subsequent year through to 2020.

Credit ratings agency Fitch on Friday upgraded Bulgaria’s long-term foreign- and local-currency issuer default ratings (IDRs) to BBB from BBB-minus.