PARIS, Oct 7 (Reuters) - France"s antitrust chief Isabelle de Silva will end her five-year mandate as head of the competition watchdog on Oct. 13, she said on Twitter earlier this week. A spokesperson for the agency confirmed de Silva"s departure, which comes as the watchdog is due to make a ruling on the proposed merger of France"s number one and two TV networks TF1 and M6. The news surprised French antitrust circles, a source close to the competition authority said, as de Silva was expected to do another term. De Silva, a high civil servant, will resume working for the Council of State, France"s highest administrative court and top legal adviser to the government. Her tenure was notably marked by hefty fines against U.S. tech giants Apple(AAPL.O) and Alphabet"s Google(GOOGL.O). The antitrust watchdog"s vice-chairman Emmanuel Combe will step in as head of the authority during the search of a new president. read more