The largest Orthodox synagogue in the West has begun requiring prospective marriage partners to show documents proving that they are Jewish.

While expressing his "personal distaste" for such a formal inquiry -- likely to sharpen the split between progressive and traditionalist Judaism -- Rabbi Maurice Lamm, the synagogue's spiritual leader, says he feels compelled by circumstances to institute the change.

Rabbi Lamm, past president of the Southern California Board of Rabbis, said he is concerned that American Jewery is being assimilated through marriages to non-Jews. He is particularly alarmed by the moves of the religion's liberal wing to proselytize non-Jews and broaden the definition of who is a Jew.

Lamm's synagogue, the Beth Jacob Congregation in Beverly Hills, is one of the largest Orthodox synagogues in North America. Lamm said he expects other Orthodox rabbis will follow suit.

He said he plans to propose to his colleagues that they adopt the Orthodox customs in Israel and Great Britain of requiring letters from prospective mates and in the case of converts, certification from recognized Jewish courts. In this country, verbal assurances usually are required.

"This will mean further division in the Jewish community -- Orthodox and Reform Jews will not be able to intermarry easily," he said. Orthodox requirements for converting to Judaism generally are stiffer than in the other branches of Judaism.

Some leaders of progressive Judaism in this country are trying to make adjustments for the growing number of marriages of Jews to non-Jews. They are encouraging converts and moving to recognize that a child raised as a Jew is Jewish, even if his mother is not. Orthodox leaders generally contend that such moves bring cultural conflicts to Jews and Gentiles and that tougher rules are the best way to fight intermarriage.