Seven MPs have walked out in protest at the Labour leader's handling of anti-Semitism and Brexit.
One of the seven, Chuka Umunna, said "a lot of Labour MPs" could follow suit, together with Tories "demoralised by the UKIPisation of their party".
Deputy Labour leader Tom Watson has warned his party could see more defections if it did not change.
He said Labour had to do more to tackle anti-Semitism and he also urged Mr Corbyn to reshuffle his shadow cabinet to reflect a wider range of MPs.
Mr Corbyn has said in a statement he was "disappointed" by the defections, which represent the biggest split in the Labour Party since the Social Democratic Party was set up 40 years ago.
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