Prime Minister Menachem Begin said today that the loss of Israeli sovereignty over the occupied West Bank and Gaza would lead inevitably to another major war in the Middle East.

Without once mentioning President Reagan's Middle East peace initiative by name, Begin made clear in a speech that his government will stake its survival on its uncompromising opposition to the Reagan plan, which calls for large portions of the occupied territories to be federated with Jordan in the future.

Begin said that as a result of Israel's defeat of the Palestinians in Lebanon, there is no threat to Israel on any of its borders, no prospect of another Arab-Israeli conflict "in the foreseeable future," and therefore no need for any new peace initiatives outside of the Camp David accords.

Opposition Labor Party leader Shimon Peres, in a sharply worded reply, accused Begin of deceiving the public by exaggerating the threat to Israel's security posed by the Reagan plan, which he said should provide the basis for negotiations on the future of the West Bank and Gaza.

Begin and Peres addressed the opening meeting of the regular winter session of the Knesset, Israel's parliament. Their speeches were designed to set the tone for the internal debate over the future of the occupied territories -- a debate that is likely to dominate Israeli politics in the months ahead.

In his 40-minute speech, Begin barely mentioned the current negotiations over the withdrawal of Israeli, Syrian and Palestinian forces from Lebanon. But urging patience in those negotiations, he vowed that Israel will not back down on its demand that all Palestine Liberation Organization guerrillas leave Lebanon before there is a complete Israeli withdrawal from the country.

In an argument that he is expected to continue to press as the debate heats up, Begin said that for Israel to give up parts of the occupied territories would be to squander its gains from the war in Lebanon.

"The question then is, why dismantle such a structure of peace?" Begin said. "Why create new conditions which would seem to invite bloodshed or war?

"Can there be any logic in inviting an enemy into our very household to disrupt the peace or prevent it? Were the program for a Palestinian state in Judea, Samaria [the biblical names for the West Bank] and the Gaza Strip to come into being, even in federation with the government of Jordan, we would again be living by our drawn and consuming sword."

Begin argued that, even if federated with Jordan, a West Bank not under Israeli control inevitably would be dominated by the PLO and armed by the Soviet Union. Such a state, he said, would attack Israel until some future Israeli government decided to act as Israel acted this summer in Lebanon.

What would follow, Begin predicted, would be another major Arab-Israeli conflict, with "all the endless bloodshed occurring only to restore the situation to what it had been" when Israel exercised sovereignty over the West Bank and Gaza.

Begin reiterated his call for the resumption of the autonomy talks called for in the Camp David peace accords, inviting Jordan's King Hussein to participate in them. "There is no reason to seek other solutions than that agreed upon at Camp David," he said.

Peres, in his reply, accused Begin of creating a deadlock in the autonomy talks through a policy of "creeping annexation" of the West Bank through the establishment of Jewish settlements in the territory.

He said the Begin government had offered Hussein no incentive to join the autonomy talks and had isolated Israel in world opinion by insisting that the West Bank is part of the historic "land of Israel," an ideological view not shared even by Israel's closest ally, the United States.

Peres said that the Labor Party also opposes creation of a Palestinian state in the occupied territories but that it does not believe the Reagan initiative would lead inevitably to that.

The Israeli debate over the Reagan plan began immediately after Begin's abrupt dismissal of the president's proposals early last month.

That debate, which was cut short by the massacre of Palestinian refugees in West Beirut, is now heating up again, but its outcome may still hinge on the Beirut massacre. A state judicial board of inquiry is to begin its investigation into Israeli responsibility in connection with the massacre this week and is expected to take several weeks, if not months, to complete its task.

The Begin government is viewed here as unyielding on the question of surrendering sovereignty over any part of the occupied territories. While the debate over the Reagan plan may flare up intermittently in the Knesset in the weeks ahead, only an extremely harsh finding by the board of inquiry is likely to threaten the Begin government's survival and therefore improve the prospects for the Reagan initiative.