LONDON, Nov 22 (Reuters) - Britain"s competition watchdog said on Monday it will not refer insurance broker Arthur J Gallagher & Co"s (AJG.N) planned $3.25 billion purchase of Willis Towers Watson"s (WTY.F) treaty reinsurance brokerage business to an in-depth investigation. The Competition and Markets Authority said it has taken its decision based on the information currently available to it. The deal, announced in August, came weeks after Willis and rival Aon Plc (AON.N) called off their $30 billion merger that would have created the world"s largest insurance broker following opposition from the U.S. Department of Justice. Treaty reinsurance involves reinsuring the whole of an insurer"s book of business, rather than individual projects or risks. Reporting by Huw Jones, Editing by Louise Heavens