With 12 MPs leaving their parties in the last week, Labour is most at risk of further splits – and the leadership seems divided on the best way to respond

When seven Labour MPs appeared at a packed Westminster press conference to announce their dramatic departure from the party on Monday, one of their colleagues was thinking about what might have been. Just two weeks earlier, the Labour MP had been all set to join them.

Already harbouring concerns about the timing of the announcement, it was a conversation with his mother that ultimately caused him to hold fire. “She just said: ‘Don’t give up your career for them. The Labour party needs sensible people to fight.’”

Over the past few weeks, many Labour MPs concerned about the party’s direction have been engaged in emotional conversations with family and friends as they try to weigh up whether they should follow suit and leave the party that most believed they would spend their lives involved in. Of the seven MPs who initially took the plunge, it was Luciana Berger, the MP for Liverpool Wavertree, who was the most surprising figure. Even in the hours before she appeared on stage, close friends struggled to believe that she would be among the group – though unreturned text messages over the weekend had made them fear the worst.

“We all put her in a different box,” said one ally. “I knew she was on the brink, but I didn’t think she’d go in their gang. Her departure has made it more difficult for a lot of us. The others had been pursuing a kamikaze strategy, but she has been pushed to this place. She gives the whole thing more weight and credibility.”

The other six MPs who left on Monday – Chuka Umunna, Chris Leslie, Angela Smith, Mike Gapes, Gavin Shuker and Ann Coffey – had been vehement critics of Jeremy Corbyn’s politics and leading supporters of a second referendum that the Labour leader has resisted. Berger, however, had been the victim of vicious antisemitic abuse.

After months of hushed conversations and secret meetings, the Independent Group (TIG) was formed, defining itself as centrist, non-ideological and internationalist. In other words, an attempted antidote to the bitter political divisions that have become a stark feature of British politics.

Such was the relative lack of organisation that they had no idea they were going to be joined soon after by an eighth Labour MP, Joan Ryan, spurred into action by Berger’s testimony. Likewise, the three Tory MPs who later joined the group had no prior warning about the timing of the Labour resignations. After a brief discussion, Anna Soubry, Sarah Wollaston and Heidi Allen announced their Tory resignations on Wednesday. By that afternoon, the new group were taking selfies as they sat together in the Commons chamber for Prime Minister’s Questions.

“That’s what brought it home for me – my friends, who often sat with me, now sitting over on different benches with former Conservatives,” said a Labour MP. “It had to happen, but it’s just really sad. That’s what I feel about it.”

We could be looking at very significant changes in the structure of British politics Lord Hutton

While the group is made up of both Tory and Labour MPs, the pool of possible Conservative defectors is very small – limited to a group of MPs who back a second referendum, or face deselection. Conversely, Labour is divided on many fronts. Many MPs are angry about antisemitism allegations and Brexit. However, the split has also refocused attention on a perceived narrowing of tolerance for political differences, resulting in a hostile party environment that MPs believe is tolerated by the leadership. The resignation on Friday of Ian Austin, a Labour MP opposed to a second referendum but unable to support Corbyn’s politics, confirmed that wider and more profound fissure.

Play Video 2:10 Jeremy Corbyn: breakaway MPs support austerity agenda – video

The Observer has spoken to Labour MPs, peers and supporters who are all on the verge of quitting. “I know personally there are up to 20 MPs sitting on the cliff edge,” said one MP. “The interesting thing is of the 20, it’s whether they jump or are kicked off.” Another said: “100%, more will go.” Several peers are also considering their position. “In many cases they’ve had 50 or 60 years of membership,” said one. “It is a wrench. However, there are people in the ‘not if, but when’ mode.”

Some MPs have had to see the funny side of the turmoil that was unleashed. “I’ve had two groups coming to me,” said one MP close to the defectors. “One group saying: ‘Come and join us, this is the new thing.’ And another saying: ‘Why don’t you fuck off and join your mates.’ But I think going would let [the leadership] off the hook.”

Many MPs said one of the most remarkable moments of the week came when Tom Watson, the party’s deputy leader, said the decision by the Labour resigners was premature. It was taken by many as tacit acknowledgement that a more serious split is likely. Prominent figures from the New Labour years believe something bigger is happening. Former cabinet minister John Hutton said: “Brexit is straining the fibres that keep our main parties together very profoundly. Depending on how things go over the next couple of weeks, we could be looking at very significant changes in the structure of British politics.”

The reaction from the Labour leader and his allies has been inconsistent. Shadow chancellor John McDonnell has attempted to sympathise with Berger’s case and suggested the party is edging towards support for a second referendum – and is fighting for changes to the way Corbyn’s office is run. There is also some action against antisemitism. The party is set to appoint Charlie Falconer, the former lord chancellor, to review its complaints procedures. The idea has been discussed for some time and was originally proposed by former Labour leader Ed Miliband.

However, Corbyn on Saturday went on the attack, expressing disappointment that former Labour MPs had opted to “join forces with disaffected Tories, who say they have no problem with austerity that has plunged thousands into desperate poverty and insecurity”. His team believes their ace card is that far from representing change, the new group actually represents a failed centre-ground politics incapable of dealing with the country’s problems.

Facebook Twitter Pinterest Tory MPs Heidi Allen, Anna Soubry andAnna Soubry and Sarah Wollaston quit the Conservatives to join the Independent Group. Photograph: Kirsty Wigglesworth/AP

There are three clear demands being presented to Corbyn by senior figures – review Labour’s approach to antisemitism, back a second referendum and head off any attempts to trigger the deselection of MPs. Three party sources told the Observer that some Corbyn allies want to begin reselections of about 50 Labour MPs by the end of the year. This allegation is flatly denied by the party.

Meanwhile, there are big hurdles for TIG MPs in the immediate future. Many Labour MPs perceived as sharing their politics have not opted to jump ship just yet, with some fellow travellers warning they will soon attempt to “put clear water” between themselves and the splitters. Ideological problems may yet emerge when the new group sits down on Monday and begins hammering out the details of becoming a coherent party – and is forced to consider thorny issues such as public spending, tax rates and welfare. While money should not be an issue for TIG – the Observer is aware of at least two former Labour donors who have already backed it – Labour will attack it for taking money from wealthy backers.

Supporters are well aware that the nature of Britain’s first-past-the-post electoral system means the group may get nowhere. “My only concern is whether this just disappears in a puff of smoke,” said one donor. “I just wonder if there’s a natural leader there. It needs someone with real clout in the party.”

Yet even Labour MPs who refused to make the jump to the new group say that it already has influence. Senior figures in the shadow cabinet now believe Corbyn’s team will try to stem the flow of defections by backing an amendment by Labour MPs Peter Kyle and Phil Wilson that would see May’s Brexit deal passed on condition that it is put to a public vote. Should the people reject the deal, Britain would stay in the EU.

“It feels like the leadership will now back the Kyle amendment and if you’d told me that just two weeks ago, I would’ve been very doubtful,” says one Labour MP. Another pro-Remain Labour figure said: “If they want to put a tourniquet around TIG so that it doesn’t affect the whole party they have to stop the bleeding soon, and that is why they are moving on a people’s vote.”And what of the Lib Dems in this fledgling realignment? For one thing, said Lib Dem leader Vince Cable, it would help if TIG had a working office and phone. “It’s a bit like the old Henry Kissinger quote: ‘Who do I call when I call Europe?’ It’s quite difficult to talk to them because they haven’t got a telephone. But I have written to them saying: ‘Let’s be sensible about this and sit down and talk.’ And I think when they have settled down next week we can have meaningful conversations.”

He had one early piece of policy advice for the group – back a new voting system. “We have had to learn to adapt to the British system, we would love to change it, and maybe this new group will be allies in helping us change that,” he said. “This new group has demonstrated there is an appetite for things to move more quickly. But they will die unless they can manage to change the system.”