Seven lawmakers on Monday split from Britain's main opposition Labour Party because they are upset with leader Jeremy Corbyn's approach to Brexit and a row over anti-Semitism.
The group of lawmakers, calling themselves "The Independent Group," included Chuka Umunna, who had at one point been seen as a future leadership candidate, and Luciana Berger, who has been outspoken about the party's approach to anti-Semitism.
"If you want an alternative please help us build it," Umunna said, speaking at a hastily arranged news conference in London. "Politics is broken. It doesn't have to be this way."
"This has been a very difficult, painful but necessary decision," Berger said.
She claimed the party had become "institutionally anti-Semitic," adding: "I have become embarrassed and ashamed to represent the Labour Party."
"I am leaving behind a culture of bullying, bigotry, and intimidation," she said.
MP Mike Gapes said, "I am furious that the Labour leadership is complicit in facilitating Brexit."
His colleague Chris Leslie said he was leaving because of "Labour's betrayal on Europe."
Corbyn said he was "disappointed" by the MPs' decision.
"Now more than ever is the time to bring people together to build a better future for us all," he said.
The quitters are only a fraction of Labour's 256 lawmakers. But this is the biggest split in the party since four senior members quit in 1981 to form the Social Democratic Party.
Allegations of Labour anti-Semitism have grown since pro-Palestinian Corbyn was elected leader of Britain's main opposition party in 2015. Corbyn has condemned anti-Semitism and acknowledged that it has occurred "in pockets within the Labour Party," but vowed to stamp it out.