Former Finance Minister Binyamin Netanyahu has said Prime Minister Ariel Sharon's Gaza pullout plan could put Israel in mortal danger.
Mr Netanyahu has been a vocal critic of the pullout plan
Mr Netanyahu, who quit the Cabinet on Sunday, said a planned port in Gaza could act as a "terrorist base" that would be a direct threat to the region.
Mr Sharon hit back by labelling Mr Netanyahu's resignation so close to the pullout as "less than honourable".
Israel plans to withdraw thousands of settlers and soldiers next week.
The well-known rivalry between the two men deepened as an opinion poll was published showing a clear lead for Mr Netanyahu among members of the governing Likud party.
The poll, shown on an Israeli television network, showed 42% of members would back Mr Netanyahu in a party leadership vote, while only 27% supported the prime minister.
An earlier poll for the Haaretz newspaper put Mr Netanyahu's support at 47% and Mr Sharon's at 33%.
In a speech to the Knesset on Wednesday, Mr Netanyahu took particular aim at the plan for Egypt to take control of its border with Gaza and the likely construction of a seaport in the territory.
"A port could represent a mortal danger," said Mr Netanyahu, who is also a former prime minister.
"We are allowing the creation of an independent terrorist base in Gaza to which arms will flow from Syria, Afghanistan, Iraq.
"We are facing a global enemy which threatens not only Israel but Egypt and Jordan as well. Who will stop this terrible danger? Apparently not the Israeli government which is blindly stumbling."
Mr Sharon responded by accusing Mr Netanyahu of shirking responsibility by choosing to leave the government so close to the start of the withdrawal.
"The only thing I can say is that to resign a week before the implementation of one of the most complex and difficult decisions ever for the state of Israel is less than honourable," he told MPs.
Mr Sharon added: "I don't know why he resigned. He supported the disengagement plan many times in the past."
The BBC's Barbara Plett in Jerusalem says opponents of the pullout have been filling the plaza in front of the Western Wall for a prayer protest.
"Some are weeping," she says. "The rabbi leading a special prayer of confession called this a time of mourning."
The Israeli government has told the 8,000 settlers and the soldiers that protect them they must leave the Gaza Strip by 17 August.
About 5,000 Palestinian security personnel are to be deployed to prevent militant attacks during the withdrawal, agencies reported.
Four small settlements in the West Bank are also to be evacuated.
Some see the Gaza withdrawal as a ploy by Israel to consolidate its grip on the West Bank, but Vice-Prime Minister Ehud Olmert rejected that.
"The pullout is in no way an attempt to trade Gaza for the West Bank," Mr Olmert, who replaced Mr Netanyahu as finance minister, told reporters.
He said he hoped the move would lead to the resumption of peace talks with the Palestinians, who he called on to "rise to this opportunity".