Four Tory MPs are expected to quit the party to join the Independent breakaway group from Labour, whose number rose to eight on Tuesday night.
ITV News Political Editor Robert Peston said the anticipated shift heightens the prospect of a general election given the weakening to Theresa May's fragile minority government.
It came as Labour announced plans, should they be elected to government, that would make it easier for constituents to force by-elections if their MP quits the party, just as MP for Enfield North Joan Ryan did on Tuesday.
She announced via Twitter on Tuesday that she was quitting Labour after four decades in protest at Jeremy Corbyn's leadership.
She follows seven other Labour MPs - Chuka Umunna, Chris Leslie, Luciana Berger, Mike Gapes, Angela Smith, Gavin Shuker and Ann Coffey - who all left Labour on Monday to join the Independent Group.
They made the decision over Mr Corbyn's leadership, his stance on Brexit and his handling of allegations of anti-Semitism.
In a letter, Ms Ryan said the decision to leave the party was "terribly difficult".
"It has been an enormous honour and privilege to serve as a Labour councillor, MP and minister over the past 25 years and I never believed I would be forced to leave the party," she wrote.
Ms Joan said that, as a child of Irish parents, she and her family had experienced first hand the issue of discrimination in the UK.
"Over the past three years, however, the Labour Party under Jeremy Corbyn has become infected with the scourge of anti-Jewish racism," she wrote.
"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 always believed that we must be especially vigilant against the oldest hatred; history teaches us the tragic and horrific results of a failure to do so.
"I have been horrified, appalled and angered to see the Labour leadership's dereliction of duty in the face of this evil."
On Tuesday, Mr Corbyn urged members of Labour to unite around his policies and expressed regret over the decision of the MPs to leave the party.
He strongly defended his policies calling for social justice and a more equal society, but acknowledged that he needed to take his party with him if they were to succeed at the polls.