MEXICO CITY, Aug 16 (Reuters) - S&P Global Ratings will lower its ratings starting point for Mexican non-bank financial institutions to BB-minus from BB, citing financing risks, the ratings agency said on Tuesday.

The situation for the institutions is becoming "more difficult" after the announcement last week that Mexican leasing firm Unifin would miss principal and interest payments, S&P said. (Reporting by Kylie Madry; Editing by Leslie Adler)