Lisa Nandy has won the backing of the Jewish Labour Movement in her bid for the Labour leadership after a heated campaign hustings.
The endorsement comes after an event on Thursday at which all four candidates apologised to Jewish members over the party's handling of anti-Semitism.
The Wigan MP said she was "honoured" to get the affiliate groups's official nomination.
She received 51% of members' votes, with Sir Keir Starmer second on 45%.
Shadow foreign secretary Emily Thornberry and shadow business secretary Rebecca Long-Bailey received 1.9% and 1.4% of members' votes respectively.
Meanwhile, Ms Thornberry faces a battle to stay in the contest by securing her place on the members' ballot due to start next week.
Ahead of a midnight deadline on Friday, she still needs endorsements from a further three local party branches to qualify for the final stage.
Her three leadership rivals have already guaranteed their places on the ballot through nominations from unions and affiliate groups.
Sir Keir, who is regarded as the frontrunner in the contest, received a further boost to his campaign by winning the backing of the TSSA transport union on Friday.
The Jewish Labour Movement, which has been a party affiliate since 1920, is one of the largest of such groups with the power to nominate leadership candidates.
The group's members can vote for whichever candidate they want when the final, one-member-one vote stage of the contest get underway next week. The result will be announced on 4 April.
During the 90-minute hustings at a synagogue in north London, all four candidates were pressed on their record on, and ideas to tackle, anti-Semitism within the party.
Labour's handling of the issue has dominated the tenure of outgoing leader Jeremy Corbyn, who is standing down following the party's defeat at December's election.
All candidates committed to implementing the recommendations of an ongoing inquiry into the issue from the Equality and Human Rights Commission (EHRC).
The watchdog launched the probe into whether the party had "unlawfully discriminated against, harassed or victimised people because they are Jewish" in May last year.
Shadow cabinet dispute
Asked whether she had spoken with shadow cabinet members about anti-Semitism, Ms Long-Bailey said she had, but added: "Was that enough? No it wasn't."
"Could I have done more? Yes."
During a BBC Newsnight debate on Wednesday, Ms Thornberry accused Ms Long-Bailey of failing to push for tougher action on the issue within the shadow cabinet.
Ms Thornberry said that along with Sir Keir, she had called for "regular reports" on cases within the party but Ms Long-Bailey had not done the same.
The shadow business secretary rejected this, responding: "I did, I think you'll find."
At the Jewish Labour Movement hustings, Mrs Long-Bailey said the party needed to put in place a "legally independent" process for handling complaints, and offer training to members on spotting "insidious" anti-Jewish conspiracy theories.
Ms Thornberry said the party was now dealing with the issue of anti-Semitism by kicking out members, but asked: "Why on earth did we not do it two-and-a-half years ago?"
She said as leader she would also be pressing for the Conservative Party to adopt a plan based on the EHRC inquiry, adding that the governing party "have issues too".
Sir Keir said getting to grips with the issue of anti-Semitism would require "strong leadership," and would be a "day one issue as far as I'm concerned".
Lisa Nandy, who quit the shadow cabinet over Brexit in 2016, said anti-Semitism had become an "existential" issue for Labour and revealed she had previously considered her position within the party over the issue.
At the scene
Analysis by BBC political correspondent Helen Catt
It was clear from the start of this hustings that the Jewish community would be looking for real answers on anti-Semitism.
In her opening speech, veteran Labour MP Dame Margaret Hodge accused the leadership candidates of "rewriting the history of the last four years," and told those who had turned out to hear them that they were owed "an explanation" for why they had not spoken up more loudly before.
Rebecca Long-Bailey was given, perhaps, the toughest time. Her claim that she spoke out about anti-Semitism was greeted with shouts of "when?".
While each candidate spoke of their shame, sadness and anger at what had happened, it is clear that the new leader will need to do much more than say sorry to repair Labour's relations with the Jewish community.
The good news for the candidates: a show of hands from the audience suggested the candidates tonight may well have made a start.
In a speech in Salford on Friday, Ms Long-Bailey set out a "path to power" for Labour, following its fourth general election defeat in a row.
She said the party had to offer "aspirational socialism", with the leadership strengthening its links with Labour local associations and trade unions.
Labour must be a "self-confident alternative" to the Conservatives, Ms Long-Bailey added.