WARSAW (Reuters) - Moody's (NYSE:) has downgraded the long-term deposit ratings of mBank and changed the outlook to negative from "under review" citing risks stemming from the legacy of Swiss franc loans at Polish lenders, the agency said.

Moody's has downgraded the long-term issuer ratings of the bank's mortgage unit mBank Hipoteczny and changed the outlook to negative from "under review".

The agency confirmed Bank Millennium's long- and short-term deposits ratings and changed the outlook on its long-term deposit ratings to negative from "under review".

An adviser to the European Union's top court last month sided with Polish borrowers with Swiss franc-denominated mortgages. Poland's regulator has warned the matter could cost Polish banks 100 billion zlotys ($22.98 billion).

"Tail risks for mBank and Bank Millennium have increased significantly in light of the opinion of the Advocate General of the European Court of Justice, which lifts the loss severity of current and future lawsuits related to the Polish banks' legacy Swiss franc mortgage loans," the agency said.

($1 = 4.3511 zlotys)