Yair Netanyahu is “spitting on the grave of his grandfather and grandmother,” Dr. Hagai Ben-Artzi, brother of Sara Netanyahu, said Monday of his nephew’s relationship with a non-Jewish Norwegian woman.

News that the prime minister’s son, who is 23, is dating Sandra Leikanger, 25, was first reported by the Norwegian daily Dagen. The tall, svelte blonde met the younger Netanyahu at the Interdisciplinary Center in Herzliya, where the two study.

Prime Minister Benjamin Netanyahu reportedly told Norwegian Prime Minister Erna Solberg at the World Economic Forum in Davos last week that his son traveled with Leikanger in Norway over the summer, and that the two had been dating for months.

In an interview with ultra-Orthodox website Kikar HaShabbat, Ben-Artzi urged his nephew to cut ties with his new girlfriend, and warned him that should he choose to pursue the relationship, Ben-Artzi would personally see to it that he would not be allowed near his grandparents’ graves.

“It’s terrible,” Ben-Artzi said. “Just terrible, and the son of the prime minister no less. It is the worst thing that is threatening and was a threat throughout the history of the Jews.”

Should his nephew marry Leikanger, Ben-Artzi said he “would bury myself, I don’t know what I would do with myself, I’d take to the streets and rip the hair out of my head — and here it’s coming true.”

If his father was alive, Ben-Artzi added, that is precisely how he would respond too.

Ben-Artzi and Sara Netanyahu have not been in touch for years for undisclosed reasons.

Earlier on Monday, ultra-Orthodox Shas MK Arye Deri responded to news of the relationship by saying, “If God forbid it’s true, then woe to us, woe to us.”

Deri told the Kol Barama radio station the relationship was no mere personal matter because Netanyahu is a “symbol of the Jewish people.”

“I know friends of mine who invest tens of millions and more, hundreds of millions to fight assimilation in the world,” Deri said.

By contrast, Rabbi Amnon Bazak of the Har Etzion yeshiva defended the prime minister and expressed the hope that should Yair choose to wed his present girlfriend, Leikanger would undergo a conversion to Judaism prior to the nuptials.

“There is no place for the harsh statements against Benjamin Netanyahu due to his son Yair’s Norwegian girlfriend,” Bezek said. “As if parents can control their children in their twenties.”

The Israeli organization Lehava, which says it aims “to prevent assimilation in the Holy Land,” called on Netanyahu on Sunday “to prevent this relationship.”

“Your grandchildren, as you know, will not be Jewish,” Lehava director Bentzi Gopshtain warned the Israeli premier in a Facebook post.

Yossi Sarid, a former Israeli education minister and onetime leader of the secular-rights party Meretz, called the younger Netanyahu’s love life a “private matter.” But he said the uproar among the religious was “nonsense.”

“It’s not fair. You can’t expect fairness from those people,” Sarid said. “They don’t like non-Jews. They don’t like non-Orthodox Jews. They are behaving as fanatics everywhere behave.”

Noah Slepkov, an associate fellow at the Jewish People Policy Institute, said the debate reflects changes taking place within Israel.

While intermarriage has long been a “huge deal” in the United States, he said, where roughly half of American Jews marry outside the faith, it has been a nonissue in Israel because Jews and Arabs almost never marry.

But that has begun to change, due to an influx of foreign workers and the trend of Israelis studying and working abroad in an age of globalization. “It’s certainly a trend that’s at the beginning,” he said, but one that nonetheless can make conservative Israelis feel threatened.

He said the criticism of Netanyahu’s son was counterproductive in a country that increasingly finds itself isolated.

“We’re insecure,” he said. “People need to realize that having a few percent of our people intermarrying is not going to hurt this.”

Yair is the son of Netanyahu’s third wife, Sara. Netanyahu was himself married to a non-Jewish woman, Fleur Cates, between 1981 and 1984.

Yair recently completed his mandatory military service in the IDF, where he served as a corporal in the IDF Spokesperson’s Unit. He currently studies International Relations at IDC.

Leikanger’s family reportedly belongs to Norway’s Evangelical community.

Netanyahu’s office refused to comment on Yair’s reported relationship.

Yifa Yaakov contributed to this report.