According to the official website of the primaries, Fillon’s rival Alain Juppe lags behind with 33.5 percent after more than 4.2 million votes were counted.
Juppe has already conceded his defeat in Sunday’s runoff of the center-right primaries and wished his challenger a victory next May.
Between 4.2 and 4.6 million are estimated to have taken part in the vote, according to an Elabe survey commissioned by the French broadcaster BFMTV.
The French presidential elections will be held in two rounds in April and May 2017. The ruling Socialist Party will choose its candidate in January, but is widely seen as an outsider due to its low popularity. Far-right Front National’s Marine Le Pen is also expected to run.