WASHINGTON, Jan 14 (Reuters) - A leading U.S. bank regulator finalized a rule Thursday that prohibits large banks from refusing to lend to certain business sectors. The rule, proposed in November by the U.S. Comptroller of the Currency, has been fiercely contested by banks who call it unworkable and political. It applies to firms over $100 billion in size, and stipulates that banks must show legitimate business reasons for not providing services to certain borrowers. (Reporting by Pete Schroeder; editing by Jason Neely)