The Israeli government was handed a detailed, written plan tonight for the withdrawal of the besieged Palestinian guerrillas from West Beirut along with a formal request that it allow a contingent of French troops to take up positions in the city before the evacuation begins.

The evacuation plan and timetable, crafted during more than 40 days of negotiations by U.S. special envoy Philip C. Habib, already have been accepted by the Palestine Liberation Organization and the Lebanese government.

Sources familiar with the negotiations said the main stumbling block to Israeli agreement on the plan was the early deployment of the French troops, which Israel has opposed adamantly on the ground that it would provide a protective "screen" for the trapped PLO guerrillas if they renege on their promise to leave Lebanon.

In Washington, the Reagan administration for the first time publicly expressed optimism about the chances for success of Habib's mission, Washington Post staff writer John M. Goshko reported.

"Our view is that there is momentum and that Ambassador Habib has made substantial progress over the last few days in working out the practical details of a PLO departure from Beirut," State Department spokesman Alan Romberg said. "We believe that if the cease-fire holds, we can have a negotiated settlement."

But administration officials privately cautioned that potential problems still remain over such issues as firm commitments on the destinations of the PLO fighters, Egypt's willingness to join the process and Israel's acceptance of French troops as the vanguard of a multinational force.

The United States is understood to have shifted its position late in the negotiations. It is said to be supporting the PLO contention that at least some part of the international force should be deployed in Beirut at the outset of the evacuation to provide protection to the guerrillas as they leave their fortified positions and begin their exodus from the Lebanese capital.

The proposal for the early deployment of 200 to 300 French troops, buttressed by regular Lebanese Army units, before the PLO evacuation begins is not expected to be accepted easily by Israel.

The first signs of the Israeli reaction may come Tuesday, when U.S. Ambassador Samuel W. Lewis is scheduled to meet with Prime Minister Menachem Begin to discuss the plan.

In what Israeli officials characterized as a major compromise, Begin yesterday informed Secretary of State George P. Shultz that Israel would allow the deployment of the full international force--to be composed of 800 French troops, 800 U.S. Marines and 400 Italian troops--after a majority of the guerrillas had left Beirut, but not before.

Elaborating on this point, Begin said in a speech last night that the international force would be deployed when only 2,000 to 2,500 of the guerrillas remained in Beirut. Although initially put at 5,000 to 6,000 Palestinians, the Israelis now estimate that there are 8,000 to 10,000 PLO fighters and Syrian troops in the city.

Begin also said Israel would demand written assurances from the United States, France, Italy and Lebanon that the international units would force defiant PLO guerrillas out of Beirut or allow the Israeli Army to drive them out. Similar assurances were understood to be in the written plan delivered to the Israeli government.

Informed sources said the PLO's ruling council and the Lebanese government had agreed to all parts of the plan, although neither has yet seen the written version presented to Israel. Habib was said to believe that speedy approval by Israel is essential to prevent the fragile agreement from unraveling and perhaps leading to an all-out Israeli attack on West Beirut.

Given Israel's previous strong opposition to the early deployment of international force units, quick Israeli acceptance of the plan as now drafted did not appear likely.

The plan does not call for any pullback by the Israeli Army during the Beirut withdrawal, but it is understood that Israeli Army units will be required to stand back temporarily from their positions at roadblocks along the Beirut-Damascus highway and at Beirut's port while PLO guerrillas are escorted out of the country.

The plan, delivered to Jerusalem by a U.S. Embassy messenger, is understood to contain a detailed timetable for the phased withdrawal of Palestinian guerrillas and the coordinated deployment of the French, Italian and American troops.

According to the plan, once Israel, the PLO and the Lebanese government give their formal approval, Habib will set a "D-day" for the departure of the guerrillas. Before that date, the first small contingent of French troops and Lebanese Army regulars will be moved in between Israeli and PLO lines.

A committee of military experts drawn from the Lebanese Army, the United States, France and Italy already has started holding meetings outside Beirut to work out details of the multinational force deployment.

Habib is understood to anticipate only a week's delay in making all of the final arrangements for the PLO withdrawal after formal agreement by the three parties.

It is expected that most of the guerrillas will leave Lebanon overland by the Beirut-Damascus highway, although vessels are said to have been chartered to evacuate some guerrillas. According to detailed transport plans already drafted, the Lebanese Army will be responsible for providing vehicles for those guerrillas traveling overland to Syria and other destinations.

Two staging areas mentioned for the dispersal of the guerrillas are the Syrian port of Latakia and Cyprus.

Not included in the plan are approximately 7,000 guerrillas being held in Israeli prison camps in Lebanon and Israel, nor approximately 14,000 PLO combatants believed to be in Syrian-controlled areas of the Bekaa Valley in eastern Lebanon. Their fate would be addressed later during negotiations for the withdrawal of all PLO forces in Lebanon outside of Beirut.

In addition to the issue of the timing for deployment of international troops, another possible stumbling block was said to be the refusal of Egypt to accept any of the Palestinians. Egypt has said it would support the evacuation plan only if the withdrawal is linked to a wider agreement committing the United States to progress on an overall Palestinian solution.

The plan delivered to the Israeli government tonight was understood to include timetables for the withdrawal of the PLO fighters to Jordan, Syria, Iraq and Sudan, but not Egypt.

But the negotiators were understood to believe that the participation of Egypt, the largest of the Arab countries, would make execution of the plan far easier. Egyptian officials are believed to be under growing pressure to reverse their position and accept up to 3,000 of the PLO fighters.

In recent days, Israeli officials have contended that it was the unwillingness of Arab countries to accept the guerrillas that stood in the way of agreement on a evacuation plan. In an interview today, David Kimche, director general of the Israeli Foreign Ministry, said, "If there is a continuation of the agony in West Beirut, it is only because of the attitude of the Arab countries."

Kimche said, "The whole thing is holding fire mainly because of the Egyptians." He also accused Syria of "playing a game" on the question of where the guerrillas would go, although Lebanese officials in Beirut said Syria was committed to accepting some of the guerrillas as outlined in the written plan.

Israeli officials are known to be aware of the shift in the U.S. position in favor of the early deployment of the French troops, but they have not publicly mentioned this as a possible final major sticking point in the negotiations.

The French troops slated for early deployment are on vessels off Cyprus. It was apparently their presence there and the change in the American stance that triggered accusations last weekend from aides to Israeli Defense Minister Ariel Sharon that Habib was conspiring with the French to allow the Palestinians to remain in West Beirut.

Sharon ordered Israeli troops to take up positions around the Lebanese port city of Juniyah, the likely landing site of the French contingent.