GDANSK, Jan 31 (Reuters) - The Polish arm of Santander Bank on Wednesday posted a 9% rise in its preliminary fourth-quarter net profit that missed expectations due to higher-than-expected legal costs for foreign currency mortgage loans.

Santander Bank Polska's quarterly net profit rose to 980.1 million zlotys , missing the 1.01 billion zlotys expected by analysts in a Reuters poll.

The lender, the country's second biggest by market capitalisation, said the cost of legal provisions for foreign currency mortgage loans was 1.01 billion zlotys compared with its earlier estimate of 856 million.

Polish banks benefited from higher rates to curb inflation, but their earnings have been burdened in recent years by costs related to Swiss franc mortgage loans, and relief measures for homeowners introduced by the government.

Net interest income jumped 21% to 3.43 billion zlotys, slightly higher than expected, thanks to high interest rates, and growth in credit portfolio.

($1 = 4.0199 zlotys) (Reporting by Mateusz Rabiega; Editing by Kim Coghill and Louise Heavens)