The German manufacturers reckons the move presents its best chance of success in the championship but that it is happy to continue to supply engines to the Williams team.
BMW has a supply deal with Williams until 2009 but the relationship between the two has deteriorated.
Williams director of engineering, Patrick Head, and BMW director of motor sport, Mario Theissen, criticised each other in the media before the Canadian Grand Prix.
Williams said it had no comment on the Sauber deal.
