Video report by ITV News Political Correspondent Libby Wiener
Long-standing Corbyn critic Ian Austin announced he is leaving Labour on Friday morning, launching a scathing attack on its leader.
The Dudley North MP condemned a "culture of extremism, anti-Semitism and intolerance" in the "broken" party and he was "ashamed" of what it had become under Jeremy Corbyn.
Mr Corbyn said he "regrets" the resignation of the ninth MP from Labour in a week, and denied there is wide-scale bullying in his Party, adding that any "bad behaviour" or "problems" are dealt with.
Mr Austin initially suggested he would not be joining forces with the group made up of eight of his former Labour colleagues and three ex-Tories.
MPs react to Austin's resignation
Asked if he would back Theresa May in any motion of no confidence, he said: "I don't think we are at that point, and I hope that that isn't the choice that faces the country in the future, but I do think that Jeremy Corbyn is completely unfit to be prime minister."
Strong division has emerged among Labour MPs following Austin's decision to quit.
The party's deputy leader Tom Watson posted on Twitter he was "very sad" to see Mr Austin leave the Party.
He said it is "personally very hard to see a close friend take a decision of this magnitude", referencing Austin's decision to quit over his lack of support for Corbyn.
Fellow Labour defectors Luciana Berger and Chuka Umunna both expressed their support for Mr Austin.
Umunna tweeted he has "massive respect" for this parliamentary colleague. He added the decision is an "incredibly difficult" one to make but that Austin "has stayed true to his values and what he believes to be the national interest."
Berger tweeted she "fully understands" why Austin has come to his "difficult and painful decision."
A tweet from the newly-formed Independent Group, which Berger and Umunna are both members of, shared the sentiment expressed by MPs.
Not all MPs are sad to see Austin quit Labour. Corbyn ally Chris Williamson, the MP for Derby North MP, described Mr Austin's departure as "no loss".
Posting on Facebook, he added Austin's "frequent ill-tempered outbursts were an embarrassment to the Labour Party," and that the MP stood as a Labour candidate in the last general election on "false pretences" to get re-elected.
Will Austin join parliament's newest group of MPs?
ITV News Political Correspondent Paul Brand says Austin rejected calls by Labour MPs for a second EU referendum, and his Brexit-backing constituents put him at odds with other Labour quitters.
These issues alone are enough for pundits to question whether he will join The Independent Group.
It was initially thought Austin would not, unlike other MPs to leave Labour, sit with the newly-formed group. The MP later clarified his standing, stating it is an issue he is considering and was spurred on to leave the party by messages from other quitters.
Corbyn and Anti-semitism Austin's reason behind resignation
Mr Austin told the Express and Star he left Labour because of his constituents, saying: "I always tell them the truth and I could never ask local people to make Jeremy Corbyn Prime Minister.
"I am appalled at the offence and distress Jeremy Corbyn and the Labour Party have caused to Jewish people.
"It is terrible that a culture of extremism, anti-Semitism and intolerance is driving out good MPs and decent people who have committed their life to mainstream politics.
"The hard truth is that the party is tougher on the people complaining about anti-Semitism than it is on the anti-Semites."
His calls to tackle the issue have been acknowledged by shadow chancellor John McDonnell.
Speaking to the Evening Standard, McDonnell said: "We've got to be quicker, and we've got to be fiercer," adding there had been a lot of listening but not a lot of action on the issue.
Austin, a stalwart among the opposition voices against Labour's leader, said: "I think Jeremy Corbyn has completely changed what was a mainstream party into a completely different party with very different values.
"The hard left is now in charge of the party, they're going to get rid of lots of decent mainstream MPs and I just can't see how it can return to the mainstream party that won elections and changed the country for the better."