Luciana Berger, the MP who quit Labour to set up the Independent Group has joined the Liberal Democrats.
The Liverpool Wavertree MP left her original party in February to form the Independent Group, which later evolved into Change UK.
Ms Berger said: "This is a moment of national crisis.
“The Liberal Democrats are unequivocal in wanting to stop Brexit and are committed to securing Britain's future as a tolerant, open and inclusive society.
"I am joining Jo Swinson and the Liberal Democrats today, in the national interest, to offer a vital, positive alternative to (Boris) Johnson and (Jeremy) Corbyn and help build a future that our country deserves."
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Liberal Democrats leader Jo Swinson tweeted: "Welcome @lucianaberger - I am so delighted and proud to welcome you to the @LibDems. So looking forward to working together to stop Brexit, fight for equality and build a fairer society for all."
Former Labour colleague Chuka Umunna, who set up Change UK with Ms Berger before switching to the Lib Dems, also welcomed the news.
"I am absolutely delighted at this news. @lucianaberger will be a huge asset to the @LibDems and is a fantastic advocate for liberal, internationalist values and social justice nationally!"
Ms Berger’s decision to join the Lib Dems comes just two days after Phillip Lee dramatically defected from the Tories during a speech by Prime Minister Boris Johnson in the Commons.