Dec 20 (Reuters) - U.S. private equity firm Warburg Pincus has partnered with Antonio Horta-Osorio, the former chair of Credit Suisse, on a bid of more than 6 billion euros ($6.57 billion) for Altice's Portuguese telecoms assets, the Financial Times reported on Wednesday citing people familiar with the matter. ($1 = 0.9139 euros) (Reporting by Anirudh Saligrama in Bengaluru; Editing by Krishna Chandra Eluri)