If Israel annexed the West Bank and Gaza Strip today -- as advocated by some ultranationalist politicians and civilian settlers -- it would become an Arab state in 20 years, and perhaps less, according to authoritative demographic projections.

In the 13 years since the 1967 War, when Israel abruptly found itself ruling more than a million Palestinians in the occupied territories after only six days of fighting, a debate has simmered beneath the surface of the Jewish state over whether to proclaim Israeli sovereignty over the conquered areas.

The debate has enormous implications for the future of Israel and, indeed, for the funamental principles of Zionism.

Among the troubling questions it raises are: did the founders of the modern Zionist movement intend that the Jewish homeland they envisioned in Palestine would become a binational state -- Arab and Jewish -- with the Jews following the South African example of a minority ruling a majority? And, would annexing the West Bank hand over to the Palestinians what they have been fighting for relentlessly for decades -- an independent state?

Hebrew University professors Dov Friedlander and Calvin Goldcheider, in a recently published treaties entitled "The Population of Israel," argue that indefinite or permanent Israeli rule over the occupied territories endangers the Jewish basis of the state of Israel.

Taking medium figures for fertility and immigration, they project that the population of the country, including the occupied areas, will be 6.7 million by 1990, and some 10 million by the year 2010. Then, they say, the Jewish population could be only 45 percent of the total.

Even allowing for a dramatic increase in Jewish immigration, the Jewish population would continue to shrink, Friedlander and Goldscheider maintain.

However, excluding the occupied territories, the projected population of Israel -- currently 3.8 million would be 6.9 million by the year 2010. Of those, 5.4 million would be Jews.

The most significant factor in the threat to Israel's Jewishness is the disparity between the annual birth rates of Jews and Arabs. It is 40 per 1,000 for Arabs living in Israel and 17 per 1,000 for Jews. In the occupied areas, the Arab birth rate is approaching 30 per 1,000, or nearly 3 percent population growth a year.

There are currently about 500,000 Arabs living in Israel proper (the nucleus having decided to remain after the 1948 war of independence), and approximately 1.2 million Palestinians in the West Bank and Gaza Strip.

One of the most significant factors of the population boom is the disproportionate number of Arabs of childbearing age. In Israel proper, 50 percent of the Arab population is under 15 1/2 years of age and 75 percent is under 30. In the last 30 years, the Israeli Arab population has tripled, to to 480,000.

Moshe Sharon, former Arab affairs advisor to the prime minister, estimates that by the 2000, the Arab population in Israel proper will approach 25 percent of the total, meaning that no Israeli government could be formed without the agreement of the Arab representation in the parliament.

Mattiyahu Drobles, chairman of the World Zionist Organization's settlement division, says he is not frightened by the apparent demographic advantage of the Palestinians.

"The Arabs in Israel have grown in number, but they are loyal to Israel. Part of the West Bank Arabs can also be loyal to Israel," Drobles said in an interview.

Agriculture Minister Ariel Sharon also appeared unperturbed by the future demography. He offered different estimates of population than those presented by Friedlander and Goldscheider, claiming that if Jewish immigration remains about 30,000 a year, Jews will make up 64 percent of the population in the area from the Mediterranean Sea to the Jordan River by the year 2000. He did not cite the source of his estimate.