But the Central Committee ballot in Tel Aviv on Thursday pushed through by hardliners opposed to the plan was non-binding.
Sharon's aides said it would have little impact since there is no majority in the Knesset (parliament) for referendum legislation.
But the vote displayed the strength of rebels in Likud who aim to join forces with the opposition in the Knesset to reject the state budget in a vote later this month.
Failure of the budget would force snap elections and could shelve the Gaza plan.
Central Committee members approved the motion for a referendum by a show of hands so overwhelming that the forum's secretary said there was no need to see who would vote no.
Sharon aims to evacuate all 8500 Jewish settlers from Gaza and a few hundred of more than 230,000 from the West Bank later this year to "disengage" from conflict with the Palestinians.
The plan, which would mark Israel's first pullout from lands it occupied in the 1967 Middle East war and which Palestinians want for a state, has been touted by US-led mediators as a springboard towards final peace negotiations.
Enjoying solid support in countrywide opinion polls for "disengagement", Sharon has rejected referendum proposals made by diehard settlers and nationalist patrons as a stalling tactic.
Likud's policy-making Central Committee is strongly opposed to ceding any of Gaza or the West Bank.