Israeli Defense Minister Ariel Sharon expressed grave concern that Christian Phalangist militiamen would massacre Palestinian civilians shortly before he ordered the Israeli Army to send the Phalange militia units into the refugee camps in West Beirut, according to an authoritative account given today.

The predictions of a potential bloodbath were described as having been made while Sharon and other senior Israeli officials were pressing for the dispatch of regular Lebanese Army units into the refugee neighborhoods to root out the remaining armed Palestinian guerrillas. They argued that the assassination of the Phalangist leader, Lebanese president-elect Bashir Gemayel, greatly increased the danger that the Christian militiamen would renew their attacks on the Palestinians.

The report of Sharon's expression of concern was provided to The Washington Post on condition that the precise details of the conversation foretelling a massacre would not be disclosed.

Sharon has publicly maintained that he and other Israeli officials never dreamed that the Phalange's militia units would go on a rampage of indiscriminate killing once they were inside the camps. But well-informed sources said the defense minister and Chief of Staff Rafael Eitan raised the prospect of just such a slaughter the night of Sept. 16.

On that night, with the blessing of Prime Minister Menachem Begin and the Israeli Cabinet, Israeli Army units in West Beirut were ordered to assist the Phalangist militiamen in entering Shatila and Sabra.

Sharon's spokesman, Uri Dan, did not return phone calls about Sharon's and Eitan's warnings.

Sources said Lebanese Army commanders were willing to order their own units into the camps but said that they could not do so without authorization from their government. They said the chief impediment to allowing the regular Army into the refugee areas was Lebanese Prime Minister Shafik Wassan, who apparently feared an adverse reaction in Lebanon's Moslem community if the Army units, generally commanded by Christian officers, were to take over the Palestinian neighborhoods.

In the end, according to sources familiar with the events leading up to the massacre, the determination of Sharon and other Israeli officials to destroy the remaining guerrillas and their stockpiles of arms overcame their fear of widespread killing by the Phalange's militiamen.

The disclosure of these fears being expressed before the massacre took place undercuts one of the principal defenses Sharon and others have offered for the decision to use the Phalange units to "clean out" the refugee camps.

In a speech to the Israeli Knesset (parliament) last week, Sharon declared, "We did not imagine in our worst dreams that the Phalangists would act in this way when they entered the battle at this stage of the fighting.

"They appeared to be a regular army in every way. They promised to fight just against terrorists."

The other main defense offered by Israeli officials for their conduct before and during the massacre is that they acted to halt the killing as soon as they learned of it. But this assertion also has been called into question by reports in the Israeli press that on Sept. 16, the first night of the massacre, the Israeli Army division command in Beirut was informed by the Phalange commander inside Shatila that "until now 300 civilians and terrorists have been killed."

Gen. Eitan later agreed to allow the militiamen to remain in the Palestinian refugee neighborhoods until the morning of Sept. 18, about 36 hours after they entered.

Israeli officials have said they sought to have the Lebanese Army take over the refugee neighborhoods, which Israeli troops were forbidden to enter because of the fear that high Israeli casualties would result. They are known to have raised this subject during meetings U.S. envoy Morris Draper held with Begin, Sharon and Foreign Minister Yitzhak Shamir on Sept. 15, and again during a second meeting Draper held with Sharon and Shamir on Sept. 17.

After the massacre became known, Eitan publicly suggested that it was "interference" by Draper that prevented the prompt dispatch of the Lebanese Army into the refugee camps. But Israeli officials have since acknowledged that they were in direct contact with the Lebanese Army before the massacre and that Draper played no role in the failure of the Lebanese Army to move into the refugee neighborhoods when it was asked to do so.

It is also clear from Sharon's public statements that Israeli military leaders planned from the beginning to use the Phalangist forces to take over the camps if the Lebanese Army did not respond promptly. According to Sharon, Eitan discussed this possibility with Phalangist commanders early on the morning of Sept. 15, even before the Israeli Army had begun to occupy West Beirut.

Some of Sharon's supporters rallied at a tribute staged for the defense minister last night in Qiryat Malakhi, south of Tel Aviv. Speaking to an enthusiastic crowd that included a number of Americans, Sharon reiterated the charge that the Reagan administration is attempting to topple the Begin government.

"Everyone should know," Sharon said, "that Jerusalem is not Saigon. It will not work here." He said that since the massacre, Israel had been subjected to a "slanderous attack" aimed at weakening it to "prepare the ground for new plans like President Reagan's plan" -- which calls for Israel to give up its control over much of the occupied West Bank and Gaza strip as part of an overall peace settlement.

Begin today met with former undersecretary of state Joseph Sisco, who is in Israel on a private visit, and reaffirmed that Israel will not even discuss the Reagan proposals, according to Israeli radio.

Meanwhile, Israeli officials confirmed that the United States has told them it has no objections to Israel maintaining military control of the strategic Beirut-to-Damascus highway during the negotiations for an Israeli and Syrian withdrawal from Lebanon.

The officials said the exact points to which Israeli troops will fall back around Beirut are still being discussed, including whether the Israelis will evacuate a communications center they have established in the Beirut suburb of Baabda, site of the presidential residence.

They also emphasized that Israel would withdraw its troops from Lebanon as soon as the Syrians and the remaining Palestinian guerrillas do the same, whether or not satisfactory "security arrangements" are made in southern Lebanon. This appeared to be a shift in emphasis by the Israelis, who prior to Gemayel's assassination had publicly stressed the need to establish a border "security belt."

In addition, the officials denied reports in the Israeli press that Israel expects Egypt to replace its popular ambassador here, Saad Mortada, with a less important and outgoing diplomat. Mortada has been recalled to Cairo for "consultations" as a result of the Beirut massacre, but Israeli officials insist they view this as only a temporary matter.