Labour leader Jeremy Corbyn has confirmed he is looking into reports that a member of the shadow cabinet sang "Hey Jews" to the Beatles song "Hey Jude".
Dan Carden has denied the story on Buzzfeed news website that he changed the lyrics of the song while on a private bus to London from the Cheltenham races in March last year.
Mr Corbyn said it was an "awful story" and "totally unacceptable" if true, but that Labour were investigating.
When asked about the song on a visit to Leeds on Saturday, Mr Corbyn said: "Dan Carden has emphatically denied that.
"It is an awful story and if it's true it is utterly and totally unacceptable. I am looking into it."
Mr Carden is seeking re-election as an MP for Liverpool Walton. He has strongly denied the allegations against him.
Mr Carden tweeted: "I have been categorical in my denial about allegations relating to a coach trip some twenty 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 allegations are the latest in a string of embarrassing stories about prospective parliamentary candidates for both Labour and the Conservatives.
A Tory candidate who made "ill-judged comments" about rape during a radio discussion stood down earlier this week.
Meanwhile a Labour candidate for Clacton pulled out after he was accused of making an anti-Semitic remark.