Lincoln Square Synagogue announced Thursday night that an anonymous donor has committed to donating $20 million, which will be used to complete construction of the synagogue’s 50,000-square-foot building on the Upper West Side. The announcement was made at a special membership meeting “to share good news.”

The gift, however, comes with a string: It is contingent on the synagogue raising an additional $3 million by April 30, 2011. The synagogue’s leadership plans to launch an aggressive fundraising effort aimed at getting 100 percent participation from its membership.

The Jewish Week reported last month that the synagogue, located at Amsterdam Avenue and West 69th Street, had halted construction on its new building due to cost overruns. Synagogue insiders estimated that the synagogue needed as much as $17 million in additional funds in order to complete the building as planned. To make up for this shortfall, Lincoln Square was considering selling naming rights to its building and sharing space with a joint venture partner.

Whether those two options are still on the table has yet to be determined.

Still, it is unlikely that the donor will seek the naming rights, as he or she has requested “absolute confidentiality,” according to Lincoln Square.

Of Lincoln Square’s 650 member families, only about 150 have been approached to help fund the new building. It is hoped that the large gift will serve as a catalyst, motivating the rest of the synagogue’s membership to contribute to the cause.

“We are exhilarated by this vote of confidence in the future of our synagogue and humbled by the size and generosity of the donation,” Rabbi Shaul Robinson of Lincoln Square Synagogue told The Jewish Week. “We thank God for having sent this extraordinary gift our way, which will enable us to continue our historical mission of outreach and to build a facility that will be a resource for the entire Jewish community.”

Questions abound regarding the anonymous donor’s identity: Is the donor a member of the synagogue? Has the donor previously contributed to the cause?

“To respect the donor’s wishes, we are asking that there be no speculation or guessing games as to the identity of this generous person,” Gloria Kestenbaum, spokesperson for Lincoln Square Synagogue, told The Jewish Week.

E-mail: Tamar@jewishweek.org