LONDON — The chief rabbi in Britain launched a blistering rebuke of the Labour Party and its leader, Jeremy Corbyn , on Tuesday, saying anti-Semitism was a “poison” inside the party and raising deep worries about the country’s “moral compass” if Labour wins next month’s elections.

In an opinion piece in London’s Times newspaper, Ephraim Mirvis pointed his finger directly at Corbyn, calling the Labour leader’s assertion that his party is confronting anti-Semitism in its ranks “a mendacious fiction.”

For years, Corbyn has been followed by accusations of anti-Semitism over statements and policies, such as support for militant Palestinian groups and his appearances on Iranian state TV — acts seen by critics as opposed to Jewish causes and concerns. Corbyn denies the claims of anti-Semitism, but they have represented a continued political liability since he took over the Labour Party in 2015.

In the upcoming election, Prime Minister Boris Johnson faces his own challenges with promises to push ahead with Britain’s divisive exit from the European Union. But Johnson also has hammered away at Corbyn’s alleged failure to confront perceived anti-Semitism in his party.

While Britain’s small Jewish population of 300,000 — in a nation of 66 million people — is fully engaged in the country’s politics, Mirvis’s comments marked a sensational intervention by a top religious leader in the Dec. 12 elections.

After the rabbi’s remarks, the Rev. Justin Welby, the archbishop of Canterbury and spiritual leader of the Church of England, fired off a tweet of endorsement, remarking on the “deep sense of insecurity and fear felt by many British Jews.”

Corbyn then fired back.

At a speech Tuesday on “race and faith,” Corbyn said: “Anti-Semitism in any form is vile and wrong. It is an evil within our society. There is no place for it — and under a Labour government it will not be tolerated in any form whatsoever.”

In the upcoming early elections — called because of stalemate in a divided Parliament over Brexit — Johnson and his Conservative Party seek a majority to finally get Britain out of the European Union.

Corbyn’s Labour Party is countering with proposals for a “radical” transformation of the economy, pushing for the nationalization of rail, mail, water and electricity grids, and a big tax bill for billionaires. Corbyn also wants to renegotiate Britain’s withdrawal agreement with European leaders and then take that deal to a second referendum over Brexit.

Since he took control of the party — and saw membership surge, driven by young activists — Corbyn and Labour have been dogged by claims of anti-Semitism.

Corbyn, alongside many in the left-wing of his party, is a strong supporter of Palestinian rights and fierce critic of Israel’s right-wing government.

But in Facebook groups, social media posts and at meetings, Labour members have been accused of going far beyond criticism of Israel and into anti-Jewish statements.

Last year, Corbyn revealed that a review of online posts among Labour members uncovered “examples of Holocaust denial, crude stereotypes of Jewish bankers, conspiracy theories blaming 9/11 on Israel, and even one individual who appeared to believe that Hitler had been misunderstood.”

Corbyn himself has been criticized for hosting a 2010 panel where Israelis were compared to Nazis. In 2012, he defended an artist’s “freedom of speech,” but failed to condemn the London mural that depicted Jewish bankers playing monopoly on a board balanced on the bent backs of the workers. In 2013, he suggested that “Zionists” do not understand “English irony,” even after living in Britain for years.

Since 2017, a dozen Labour members of Parliament have quit the party, in part over its handling of anti-Semitism (alongside Corbyn’s leadership and the party’s leftward tilt).

Earlier this month, the Jewish Chronicle, which has campaigned against Corbyn, published a front-page editorial against the Labour leader and released results of a poll claiming 47 percent of British Jews would “seriously consider” leaving Britain if Corbyn won next month.

Roughly, in the post-World War II years, Jews voted heavily for Labour; then were wooed by the Conservatives during Margaret Thatcher’s leadership in the 1980s; then came back to Labour under Tony Blair; and now are running away from Corbyn’s party.

The senior rabbi, however, left no doubt where he stood. He called Corbyn unfit for leadership and even questioned the place of Jewish life in Britain under a Labour government.

“It is a failure to see this as a human problem rather than a political one,” wrote Mirvis, chief rabbi of the United Hebrew Congregations of the Commonwealth.

“It is a failure of culture. It is a failure of leadership. A new poison — sanctioned from the top — has taken root in the Labour party,” he added.

Mirvis, the spiritual leader of Britain’s 62 Orthodox synagogues, suggested he did not want to inject himself into the campaign, but insisted the “very soul of the nation is at stake.”

“It is not my place to tell any person how they should vote,” he wrote. “I regret being in this situation at all. I simply pose the question: What will the result of this election say about the moral compass of our country?”

Julia Bard, a Labour Party activist from Corbyn’s London Islington neighborhood, who is Jewish, said: “I know Jeremy Corbyn. There’s not an anti-Semitic cell in his body.”

Bard, who serves on the editorial committee of the Jewish Socialist journal, said the rabbi’s charges against Corbyn were false — and politically motivated.

“He’s not elected by anyone, he’s appointed,” Bard said of Mirvis. “He represents a section of the British Jewish community, not all Jews in Britain.” She said, “He supports the Conservative Party.”

Bard said she was very comfortable in the Labour Party — and she said more worried about the Conservative Party’s alleged biases against Muslims and black Britons.

Berger reported from Washington.