A member of Labour's shadow cabinet has denied singing "Hey Jews" to The Beatles' song Hey Jude on a coach trip last year.
Shadow international development secretary Dan Carden was accused of singing an altered version of the song on a journey back from Cheltenham Festival, BuzzFeed News reported.
Jeremy Corbyn said: "If it's true, it is utterly and totally unacceptable."
Mr Carden said he stood by his record as an anti-racist campaigner.
BuzzFeed News journalist Alex Wickham claimed he was sitting behind Mr Carden on a "private bus" in March 2018, along with other Labour MPs and MPs from other parties.
He said Mr Carden, who is seeking to be re-elected as MP for Liverpool Walton, "repeatedly sang the chorus of 'Hey Jude', replacing the word 'Jude' with 'Jews'."
Labour leader Mr Corbyn said he was "looking into" the allegation.
In a Twitter thread, Mr Carden said: "I have been categorical in my denial about allegations relating to a coach trip some 20 months ago.
"This was a coach full of journalists and MPs. If anyone genuinely believed any anti-Semitic behaviour had taken place, they would've had a moral responsibility to report it immediately.
"Yet this allegation is only made now when a general election is imminent.
"I stand by my record as an anti-racist campaigner. I would never be part of any behaviour that undermines my commitment to fighting racism in all its forms."
The news website said it was "choosing to publish" the story now after "fresh anti-Semitism allegations against Labour candidates over the last 48 hours".
A Labour candidate in Aberdeenshire quit on Thursday after the Jewish Chronicle reported that she compared Israel to an abused child who becomes an abusive adult.
And another Labour candidate pulled out of the election race on Friday over the use of an anti-Semitic remark.
On Thursday, the Labour leader told the BBC "anti-Semitism is a poison and an evil" and insisted his party had confronted anti-Semitism and taken action.
Mr Corbyn said members had been suspended or expelled and an education programme had been set up.