Israel's ambassador-designate to Cairo, Eliahu Ben-Elissar, sharply criticized both Egypt and the United States today for attempting to meddle in Israel's policy of setting Jewish civilians in the occupied West Bank and Gaza Strip.

Practically on the eve of the official opening of Israel's embassy in Cairo, Ben-Elissar warned Egypt and the United States not to invoke the settlement issue on the "pretex" that it aggravates instability in the Middle East.

Israel, Ben-Elissar said, will continue to settle Jews throughout the occupied territories, including the exclusively Arab city of Hebron, because it has an undeniable right to do so.

Ben-Elissar also suggested that the United States was inching toward the Arab position of modifying U.N. resolution 242, which calls for Israeli withdrawal from territory occupied in the 1967 war but treats the Palestinian issue as merely a refugee problem and makes no reference to selfdetermination for inhabitants of the West Bank and Gaza.

By neglecting to oppose positions taken on resolution 242 by Great Britain and European Economic Community nations, Ben-Elissar said, the Carter administration was lending tacit support to amendments in the document that would adversely affect Israel's national security.

At a meeting with foreign correspondents here, Ben-Elissar said, "the United States has committed itself, especially in signed documents [by] several presidents of the United States, to act against any modification of resolution 242, and I absolutely do expect the United States government to honor their signatures."

Amid increasing criticism of Israel's settlement policy by the Carter administration and by Egypt, Ben-Elissar said that defending what he termed the historical Jewish right to live in the West Bank "might raise a lot of difficulties, and we shall live with them.

"But you do not sacrifice vital national interests for a more comfortable diplomatic life," he added, meaning with Egypt.

When asked whether Israel's settlement policy represents a political liability in the face of newly established normal relations with Egypt, Ben-Elissar replied:

"I deny any right of any foreign power to intervene with our policy of settlement. Only those powers which want to see us withdraw from Judea and Samaria to the 1967 boundaries want us to stop having settlements." Judea and Samaria are the Biblical names for the West Bank.

"The peace treaty between Egypt and Israel is not an initiative that should bring national suicide to Israel," he added.

While the views of Ben-Elissar, a loyal follower of Prime Minister Menachem Begin who describes himself as a "revisionist Zionist," are well known in Cairo and Washington, observers here were surprised he would be so outspoken shortly before assuming his duties as Israel's first envoy to Egypt. Israel's embassy in Cairo opens officially on Monday, and Ben-Elissar is scheduled to present his credentials on Feb. 26.

Ben-Elissar appeared to be irked by reporters' repeated questions about settlements, saying at one point, "I really can't understand, why does the American administration have the right to tell the government of Israel where [a Jewish settler] has a right to live?"

He said the "whoever doesn't allow a Jew to settle where he wishes has one twisted mind."