One of the world’s leading Jewish demographers says the number of Jews in New York is really about 100,000 less than indicated by the new Jewish Community Study of New York, commissioned by UJA-Federation.

The New York study, released earlier this month, determined the Jewish population of the five boroughs, Long Island and Westchester to be 1.54 million.

But Sergio Della Pergola, professor of population studies at the Hebrew University in Jerusalem, told The Jewish Week that individuals who say they are Jewish yet do not have Jewish parents and who profess another religion should not be included as Jews.

He said he based his position on the Israeli Supreme Court decision in the famous Brother Daniel case a half century ago in which a Catholic priest, born of Jewish parents, sought to be recognized as a Jew in Israel. The Court ruled against him.

Della Pergola said the issue was significant because by his criteria, the Jewish community has not grown in the last 20 years but rather “has remained stable.”

Steven M. Cohen, a demographer who helped conduct the survey, said the number of people Della Pergola is referring to is closer to 19,000, or about 1.2 percent.

Jack Ukeles, head of Ukeles Associates, which helped conduct the new study, said that he and his fellow researchers interviewed a number of people who said they were partially Jewish, and others who first identified themselves as Jewish and in the course of the interview said they belonged to another religion. The “vast majority” of these people, Ukeles said, were children of intermarriages.

“People have to realize that we are living in a different world, one where a lot of people have some connections to Jewish life. We came across hundreds of borderline cases. This is no longer a matter of black and white,” he noted, “not even gray.”

He said he and his associates “agonized” over each of the complex cases and used a set of criteria to determine whether to include them as Jewish or not.

Ukeles said the percentage of such cases was very small, but showed a significant increase over the last New York population study a decade ago.

“It’s a very important issue and one that will only grow,” he said. “We as a community have to determine how we relate to these people who value their Jewish connection.”