GDANSK, Oct 31 (Reuters) - One of Poland's top lenders mBank reported bigger than expected third-quarter loss on Tuesday, hurt by provisions related to mortgages tied to Swiss francs.

Like some other Polish banks, mBank has been grappling with the legal fallout from Swiss franc mortgage lending, forcing it to boost its provisions that weigh on its earnings.

Commerzbank's Polish unit said its quarterly net loss narrowed to 83 million zlotys ($19.8 million) from 2.28 billion last year, but missed analysts' forecast for a loss of 78 million zlotys.

Poland's fifth biggest lender by market value, mBank was the first one among its peers to post a quarterly loss, after rivals Santander BP, Alior Bank and Bank Millennium all boasted profits.

The bank's quarterly net interest income rose more than threefold to 2.28 billion zlotys, supported by interest rates hikes, driving its net interest margin up to 4.26% from 4.03% a year earlier.

mBank, which is among the lenders with the biggest portfolios of Swiss franc mortgages, said provisions for legal risk of foreign currency loans stood at 1.08 billion zlotys in the quarter, down from 2.31 billion in the same period last year.

It added that 85.6% of its portfolio was now covered.

The lender also said it expected a slightly negative impact from recent interest rate cuts on its net interest income and margins in the near term.

Poland's central bank started lowering rates last month after keeping them elevated for over a year. ($1 = 4.1952 zlotys) (Reporting by Mateusz Rabiega and Anna Pruchnicka; editing by Milla Nissi)