“It is painful. It is a horrible place to be and it can’t go on,” said the former chairman of the Commons Foreign Affairs Committee.
And, hinting he was ready to walk out, Mr Gapes added: “Something has to change in the party or everyone has to make their own position, about where they stand. I am agonising with this every day.”
However, he refused to comment on a report that he had told fellow Labour MPs “it’s over for me” – implying that his resignation was certain.
Up to a dozen Labour MPs are thought to be contemplating breaking away, prompting warnings from Corbyn loyalists that a split would make it easier for the Tories to cling onto power.
Mr Gapes’ comments follow the widespread criticism of Mr Corbyn’s remarks at a pro-Palestinian event five years ago.
The Labour leader criticised “Zionists” who “don’t want to study history, and secondly, having lived in this country for a very long time, probably all their lives, don’t understand English irony either”.
Some Labour MPs reacted with horror while a leading Jewish charity accused Mr Corbyn of “unambiguous antisemitic hate” and Conservative MPs reported him to the parliamentary standards watchdog.
In a statement, he claimed he was “now more careful with how I might use the term ‘Zionist’ because a once self-identifying political term has been increasingly hijacked by antisemites as code for Jews”.
Mr Gapes, the MP for Ilford South, in northeast London, responded to Mr Corbyn’s video in a post in a Labour grouping on the WhatsApp messaging service, apparently saying a personal “red line” had been crossed.
The MP declined to comment on what he called a “leak”, but told The Independent: “I agonise every day about the state of our party.”
Mr Umunna, meanwhile, the MP for Streatham in south London, said “people from all parties and people of no affiliation at all” were campaigning together for a further referendum.
“The idea that the People’s Vote campaign is a precursor to a new party is complete and utter bollocks,” he said. “Frankly, people need to stop spreading false news about this.”
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