Joan Ryan has become the eighth Labour MP to quit the party in the past 48 hours, citing its tolerance of a “culture of anti-Jewish racism”.
The Enfield North MP said she was “horrified, appalled and angered” by Labour’s failure to tackle anti-Semitism, saying its leadership allowed “Jews to be abused with impunity”.
Ms Ryan said she did not believe Jeremy Corbyn was fit to lead the country.
Seven other MPs quit on Monday to form the Independent Group in Parliament.
There is mounting speculation that a number of Conservative MPs disillusioned with the government’s policy on Brexit could join forces with them.
One source has told the BBC’s assistant political editor, Norman Smith, to “be on standby mode”.
BBC Newsnight’s political editor Nick Watt said Conservative whips were reporting three MPs – Sarah Wollaston, Heidi Allen and Anna Soubry – had gone “very, very silent”.
While the Independent Group are not confirming anything, he said he had been told by one member that Wednesday would be a “very busy day”.
Announcing her decision on Twitter, Ms Ryan said she would continue to represent the north London seat in Parliament.
On Tuesday, she told BBC Radio 4’s Today programme that she would not trigger a by-election in her constituency, as she won her seat in 2017 “in spite of [Mr Corbyn], not because of him”.
“I didn’t win my seat on his coat tails,” she added.
Ms Ryan, who served as a minister under Tony Blair, follows Chuka Umunna, Mike Gapes, Luciana Berger, Ann Coffey, Angela Smith, Gavin Shuker and Chris Leslie in quitting the party.
In her resignation statement, she said Mr Corbyn and the “Stalinist clique which surrounds him” was not providing real opposition at a moment of crisis for the country.
Instead, she said the leadership was focused on “purging their perceived ideological enemies within and obsessing over issues of little interest to British people”.
Ms Ryan, chair of the Friends of Israel group, repeated Ms Berger’s claim that the party had become “institutionally anti-Semitic”, suggesting that under Mr Corbyn’s leadership Israel had been “singled out for demonisation and de-legitimisation”.
“The Labour Party under Jeremy Corbyn has become infected with the scourge of anti-Jewish racism. The problem simply did not exist in the party before his election as leader.
“No previous Labour leader would have allowed this huge shame to befall the party. I have been horrified, appalled and angered to see the Labour leader’s dereliction of duty in the face of this evil.”
Ms Ryan lost a non-binding confidence vote of her party members in September which she blamed on “Trots, Stalinists, Communists and the assorted hard left”.
Conservative MP’s concern
Conservative MP Phillip Lee told BBC Radio Berkshire he had considered leaving his party.
He said: “I do feel like my party is drifting from beneath me. There is this danger of some form of ‘Ukip-lite’ party developing… and I don’t remember a vote of the parliamentary party to become the Brexit Party.
“So yes, I’d be lying…if I hadn’t considered all these things. But my own firm belief is…the Conservative Party has always been a broad church. I’m going to stand and fight until it ceases to be so.”
Members of the Independent Group, who have cited what they say is a culture of bullying in the party and Labour’s stance on Brexit for quitting, welcomed Ms Ryan’s decision to join them.
Mr Shuker, the MP for Luton South, tweeted that the group was “building something powerful together”.
The seven have said their grouping could be the basis for a new political party and have urged like-minded MPs from other parties to join them.
Mr Corbyn has said he wants to “take MPs with him” but insisted that the direction he has taken the party in since 2015 is hugely popular within the country.
Chris Williamson, the MP for Derby North, said he was “not entirely surprised” by Ms Ryan’s exit.
“She was probably facing a de-selection in any event,” he told BBC’s Newsnight.
He said he had never known Labour to be “more united” than it was now and it was “regrettable that a minority of MPs” were out of step with the popular mood in the country.
Labour has suggested MPs who change political allegiance have a duty to seek a fresh mandate from their constituents.
The party is considering giving voters the power to force MPs who switch parties between general elections to face by-elections by strengthening the existing recall laws.
In a statement released before the news of Ms Ryan’s exit, shadow Cabinet minister Jon Trickett said voters should not have to wait years to hold to account MPs who they believe are not “properly representing their interests”.