Jewish voters have deserted Labour in the wake of the anti-Semitism row, John Mann MP has said.
Mr Mann, who confronted Ken Livingstone over comments he made about Hitler, said Labour should have been enjoying a "landslide" victory.
He told the BBC: "It is clear that the Jewish vote, which for generations in families has been Labour, has gone against Labour tonight. We have seen that in Bury, we have seen it in Glasgow, I think we will see it tomorrow in parts of London.
"This is a huge challenge, it's going to define Jeremy Corbyn's leadership. He has to lead from the front and get on top of this problem."
But the Labour MP insisted "now is not the time" for any challenge to Mr Corbyn.
Labour leader hits out at a harmful "running commentary" in the media as he admits the party is not in a position to win the next election.
MPs say the results show that Jeremy Corbyn's brand of socialism isn't working - but he has done well enough to stave off a coup.
A Labour MP opposed to Jeremy Corbyn just texted me: he has until next year to prove he can win the General Election in 2020.