United Synagogue is a union of British orthodox Jewish synagogues, representing the central orthodox movement in Judaism. With 63 congregations, comprising 80,000 members, it is the largest synagogue body in Europe. The spiritual leader of the union bears the title of Chief Rabbi of Britain and the Empire - a title that bears some formal recognition by the Crown, even though his rabbinical authority is recognized only by slightly more than half of British Jews.
The United Synagogue was mandated by an act of Parliament in 1870, granting formal recognition to a union of three London synagogues forged by Nathan Marcus Adler, who bore the title of Chief Rabbi of Britain. Leaders of the organization included Nathan Rothschild, 1st Baron Rothschild, who served as president in 1910.
At the time of its inception, the United Synagogue was the dominant force in Jewish communal and religious organization. With mass migrations of Jews from Eastern Europe starting in the 1880s, the United Synagogue lost some of its hegemony. The immigrants brought with them strains of Hassidic Judaism, as well as Reform Judaism and secularism.
In 1887, Jewish community leader Samuel Montagu created the Federation of Synagogues, which worked to unite orthodox synagogues of Russian and other eastern European migrants in the Jewish slums of London. Today, the Federation serves 21 synagogues, compared to the United Synagogue's 63. There are also numerous orthodox synagogues in Britain, including Haredi, Chabad, and others, unaffiliated with United Synagogue. In addition, there are congregations of Reform, Conservative and progressive Jews that are not included in the United Synagogue; so that, today, the organization represents about 30 percent of all British congregants. Since 1990, central orthodoxy has declined from 66 percent to 55 percent of total congregants, though this decline has flattened out in recent years.
United Synagogue provides a number of religious services to the orthodox community, including:
Activities are financed from dues paid by member synagogues, as well as from revenues from the organization's GBL 250 million in assets (mostly synagogue buildings).
United Synagogue is an active supporter of Israel, in particular Zionism. The United Synagogue affirm on their website  that:
The organization sponsors trips to Israel for members and youth, distributes information packages about Israel from its website, and offers courses in Israeli history and politics and Hebrew.
The organization was called a "Zionist pressure group" by PressTV, the English-language news service of Iran for its decision to press their Members of Parliament to reject  or make amendments to an October 2014 UK parliamentary motion, proposed by Labour MP Grahame Morris, which calls on the UK government to symbolically recognize the state of Palestine. The United Synagogue led its pre-Yom Kippur message to congregants with this issue.