The Israeli Cabinet voted unanimously today to approve the second stage of Israel's planned three-stage withdrawal from southern Lebanon and ordered the Israeli Army to begin the pullout immediately.

The decision was supported by Foreign Minister Yitzhak Shamir, the leader of the right-wing Likud bloc in the national-unity government here, who said he reversed his earlier vote opposing the withdrawal plan because there was "no use in making proposals without any chance of getting them through the Cabinet."

But Shamir stressed, in an interview with The Washington Post, that he still had doubts about the timing of the withdrawal and indicated that he was reserving judgment on supporting the final stage of withdrawal to the international border until he learns how the Army proposes to protect civilians living in northern Israel from terrorist attacks.

"I hope the Army will find an answer to this before we reach the international border," Shamir said. "We still have a few months."

Shamir, prime minister in the previous Likud government and due to return to that office in 18 months under the coalition agreement struck with the Labor party, also made these points in the hour-long interview:

He said the retaliatory raids in southern Lebanon against Shiite villages were intended to give the Shiites "a lesson for the future. The Israel Defense Force is not a defeated army. The decision to withdraw is not a decision of weakness."

Shamir said he "would like to believe" that Egyptian President Hosni Mubarak is sincere in offering new peace-talk proposals, but he added, "We are not naive. We know that he is going to the United States" next week to seek new aid from a Congress concerned about the lack of progress on Egyptian-Israeli issues.

He predicted that Egypt and Israel would reach agreement "very soon" on an interim status for the disputed Gulf of Aqaba beach territory of Taba and suggested a ministerial-level conference to try to reach a "package deal" that would resolve the final status of Taba and all other outstanding Egyptian-Israeli bilateral disagreements.

Shamir warned that U.S. failure to provide $800 million in supplemental aid requested for fiscal 1985 would have very grave effects on the Israeli economy, which is beginning to respond to the belt-tightening measures the coalition government has implemented since taking office in September.

Shamir, a senior member of the Likud government that approved the invasion of Lebanon in 1982 and that did not draw up its own withdrawal plan, had joined five other Likud members of the Cabinet in voting against the withdrawal plan presented on Jan. 14. Sixteen ministers voted for the three-stage plan.

The second stage in the withdrawal plan that was authorized today calls for the Israeli Army to evacuate positions in eastern Lebanon and to pull back to a line that runs just north of the town of Hasbayya. The first stage of the withdrawal, in which Israeli troops left the Lebanese port city of Sidon and the surrounding area in the west, was accomplished on Feb. 16.

The first-stage pullback was followed by an escalation in the attacks on Israeli soldiers by Lebanese Moslem Shiite guerrillas in the area still occupied by Israel. The Israeli Army has responded to the attacks with a severe crackdown during which a number of persons have been killed, villages have been searched, houses destroyed and dozens of persons arrested.

These first- and second-stage pullbacks still will leave the Israeli Army well north of the border, preventing any immediate threat to northern Israel. Thus, the second-stage vote was neither as risky nor as controversial as will be a decision to complete the withdrawal plan.

Nevertheless, today's unanimous Cabinet vote was evidence of the strong sentiment in Israel to complete the withdrawal as soon as possible. The only indication of continuing opposition came from two Likud former defense ministers -- Ariel Sharon, now the minister of industry and trade, and Moshe Arens, now a minister without portfolio -- who did not vote on the issue, according to a senior government official.

Sharon, however, told an audience in Jerusalem tonight that he had voted for the second stage of the withdrawal plan. There was no immediate explanation for the differing versions of his vote.

The Cabinet set no date for completion of the second-stage pullback, but the tentative target has been around April. A senior official told reporters that the Army "has a free hand" in executing the withdrawal, which will be affected by such factors as the weather in the mountains of eastern Lebanon that are now covered with snow.

"The pace depends on the Army," the official said. "It does not depend on politics." The official added that Israel "will make no attempt to coordinate" this portion of its withdrawal plan with the Lebanese.

The Cabinet today also began a discussion of Mubarak's proposals of this week but reached no decision on a formal government response to them. The discussion is expected to continue at a later Cabinet meeting.

In the interview, Shamir, who spoke with animation, said he would not criticize Prime Minister Shimon Peres' initial favorable response to the Mubarak proposals. From the outset, Shamir has displayed a good deal more skepticism toward Mubarak's initiative than has Peres.

Shamir insisted that the Egyptian envoys who visited Jerusalem last week had not presented formal proposals to the Israeli government but merely had engaged in general discussions about Egyptian-Israeli relations.

In interviews with U.S. and Egyptian newspapers last week, Mubarak outlined proposals for involving Palestinian representatives in peace talks that would include, in different stages, Jordan, Egypt, the United States and Israel.

Peres responded favorably to a report that said Mubarak had called for direct talks between an Arab delegation and Israel. But he rejected Mubarak's subsequent formulation calling for a first stage of talks between the United States and a delegation of Jordanians and representatives of the Palestine Liberation Organization.

Shamir insisted that any new negotiations have to be conducted under the terms of the Camp David peace accords, which call for a five-year interim period of autonomy for West Bank Palestinians before the final status of that territory is decided. King Hussein of Jordan has already rejected negotiations on that basis.

But Shamir seemed to hold out more optimism that the impasse Egypt and Israel have reached on the future of the Taba resort territory can be resolved in the near future as part of a general improvement of Egyptian-Israeli relations. "We have paid a heavy price for peace with Egypt," he said. "We have to give some substance to this peace. It has to become a real peace."

Peres has sounded more forthcoming in his comments about the possibilities of referring Taba to international arbitration. Shamir emphasized that he did not want Taba singled out as a separate issue and said that it should not be "the price" Israel paid for a summit meeting between Peres and Mubarak.

He said technical-level Egyptian and Israeli delegations would meet again soon in the Egyptian city of Ismailia to arrange for an interim status agreement on Taba while the larger question of sovereignty is sorted out.