Israel Radio said on Monday that Likud's Central Committee voted by 51% to 49% against Netanyahu's motion to advance a party leadership ballot to November in protest at the Gaza Strip pullout Sharon championed. The primary is scheduled for April.
However, the poll had a margin of error of 5%.
Sharon's aides had said if the prime minister lost Monday's vote, he might bolt the Likud and create a centrist bloc capitalising on majority popular support for the withdrawal that Israel completed two weeks ago.
Official results were expected at around midnight.
Netanyahu argued the pullout, the first evacuation of Jewish settlements from land Palestinians want for a state, would encourage resistance movements.
Sharon called the move out of Gaza an attempt to disengage from conflict with the Palestinians and leave occupied territory he said Israel had no chance of keeping under a future peace treaty.
But the 77-year-old Israeli leader has pledged to hold on forever to large illegal settlement blocs in the occupied West Bank, where 245,000 settlers live in isolation from 2.4 million Palestinians.