There have been growing reports a group of disaffected Labour MPs could quit the party due to their disgruntlement with Labour leader Jeremy Corbyn's stance on Brexit, as well as his handling of antisemitism allegations.
MPs Luciana Berger, Chris Leslie, Angela Smith, Gavin Shuker, Mike Gapes, Ann Coffey and Chuka Umunna all quit the party, with Ms Berger saying Labour had become institutionally anti-Semitic and that she was "embarrassed and ashamed" to stay.
Another of the resigning MPs, Mike Gapes, wrote a letter to his constituents - in the seat he has held for Labour for 27 years - that raised Mr Corbyn's anti-US foreign policy, the anti-Semitism issue, and Brexit.
Corbyn denies that he has allowed anti-Semitism to grow in the Labour Party and has pledged to stamp it out.
The Jewish MP, who is nine months pregnant, has been threatened with deselection by her local party after speaking out against her party's leader.
The backlash was swift after the so-called "Gang of Seven" announced their decision, with members of the shadow cabinet accusing them of launching a "direct attack on the Labour Party".
"It's very cold out there as an independent", she said.
"We represent different parts of the country, we are of different backgrounds, we were born of different generations, but we all share the same values".
McDonnell also re-assured the anti-Brexit MPs who are considering quitting the party that Labour could still yet support a new referendum, or what campaigners call a "People's Vote", despite Corbyn's reservations.
She told Sky News' Sophy Ridge on Sunday show: "There are people, I know, who have been working desperately hard - people outside the Labour Party - who have been working hard for years to try to persuade the Labour Party to split".
There have been fears, many publicly articulated but expressed with more fury in private, about the leadership's attitude to security policy, to North Atlantic Treaty Organisation, as well as Jeremy Corbyn's response to the Skripal attack and his attitude to the Trident nuclear deterrent.
"The Corbyn government would threaten our security and worldwide alliances".
If more Labour MPs join, pro-EU Tories could be tempted to follow if they feel there's potential to create a legitimate power in the center of British politics.
The internal party motion passed at Labour's weekly parliamentary meeting in the lower house, escalating internal rifts over the issue.
"Any criticism of the leadership is responded to with abuse and accusations of treachery". "Anti-Semitism is rife and tolerated".
Some of them will argue that if other MPs join them they can build a group that exploits the hung Parliament as effectively as the DUP has.
Especially if what you get caught saying is a pretty blatant - and accurate - statement about national politics, and you also happen to drop the f-bomb.
But their move underlines the increasing frustration within Labour over Corbyn's reluctance to change his Brexit strategy - the leftist leader and long-time critic of the European Union has stuck to his preference for a new election or his plan to leave the bloc.