Uncertainty over Catalonia's political future is set to hurt the independence-minded Spanish region's public finances after an international agency lowered its credit rating.

Standard & Poor's downgraded Catalonia's rating from BB to BB- due to the "nascent tensions" between the region and the central government in Madrid.

S&P said in a statement late Friday it maintained a "negative outlook" for Catalonia because the political controversy could make it hard for the region to borrow money.

It also noted that the northeastern region, which represents nearly a fifth of Spain's economic output, has a high debt load and a poor budgetary performance.

A pro-independence alliance won most votes in a Catalonia regional ballot last month, fueling the secessionist drive.

Spain has ruled out any possibility of Catalonia becoming independent.