"What impact do you think that might have?"
The question appalled comedian David Baddiel, who tweeted he felt a "terrible chill" at hearing it said "with no sense of WTF".
Actress Tracy-Ann Oberman said she "couldn't quite believe I was hearing it".
When asked for comment, a BBC spokeswoman said Mr Webb was trying to highlight "a growing trend among US Democrats of suggesting that antisemitism is not comparable with other forms of racism".
She told the JC: “Justin is the first to admit he should have phrased his question better.
"He intended to highlight a growing trend among US Democrats of suggesting that antisemitism is not comparable with other forms of racism; he was certainly not expressing any personal view.”
The JC understands Mr Webb spoke privately with Mr Baddiel on Tuesday. A BBC source said the comedian "accepted Justin meant no harm".
The source added Mr Webb accepted the question needed to be better phrased because "the audience will not have been as familiar with the state of American politics as he and John Zogby are".
