Former Labour frontbencher Luciana Berger has joined the Liberal Democrats.
The Liverpool Wavertree MP left Labour in February to form the Independent Group, which later evolved into Change UK.
But she left that party in June and has now joined Jo Swinson’s Lib Dems.
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.”
Her 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.
Ms Swinson said: “Luciana is a passionate advocate for women’s and LGBT+ rights, and she’s at the forefront of efforts to counter anti-Semitism and prevent discrimination.
“I’m delighted to welcome her to the Liberal Democrats, where we can work together to stop Brexit and build a fairer, more equal society for all.”
Chuka Umunna, who followed the same route to the Lib Dems as Ms Berger, joined the party in June and in August Sarah Wollaston joined via the Tories and Change UK.
Ms Swinson said: “Luciana becomes the fourth MP in three months to cross the floor and join the Liberal Democrats. We’re thrilled to add her perspective, expertise and skills to our ever-growing parliamentary team.
“The Liberal Democrats are growing in strength as we lead the fight to Stop Brexit altogether.
“We are fully behind a People’s Vote, and we are the rallying point for Remainers and the liberal centre ground.”