Founding member of Change UK and former shadow business secretary Chuka Umunna has joined the Liberal Democrats.
In a statement, the Remain-supporting MP said he wanted to be at the forefront of a progressive and internationalist movement in British politics.
He said: Labour and the Tories are committed to facilitating Brexit, and Brexit makes ending austerity virtually impossible.
I am convinced the Liberal Democrats, as the spearhead of a broader progressive movement in civil society, offer the best chance to improve the lives of those I represent as well as countless other citizens across our country.
Lib Dem leader Sir Vince Cable welcomed Mr Umunna into the fold.
He said: Chuka and I have worked together effectively for many months, campaigning for a People’s Vote and to Stop Brexit.
I know that he will be a great asset to our party not just on Brexit, but in fighting for the liberal and social democratic values that we share.
He joins alongside 20,000 people across the country just this month, demonstrating clearly that the Liberal Democrats are the biggest, clearest and most formidable force in the liberal centre-ground of British politics today.
But Labour Party chairman Ian Lavery called for a by-election in Mr Umunna’s Streatham seat, tweeting: Three parties in as many months….who’s next?
Put your immense popularity to the good people of Streatham…let’s have a PV (People’s Vote) on you and your principles. #letsgoforit #byelectionnow.
In February the former Labour leadership candidate dramatically quit the party along with six of his colleagues to form The Independent Group.
They were furious at Jeremy Corbyn’s leadership and what they saw as the party’s move to the hard left. They also criticised his handling of antisemitism within Labour.
At the time Mr Umunna said it was time we dumped this country’s old-fashioned politics and broken political system to give this country a politics fit for the here-and-now.
But, despite a slick launch, the party was beset by problems. First, one of its MPs described people from an ethnic minority as having a funny tinge.
Then it was unable to register its name with the Electoral Commission. The group became Change UK.
In the recent European Election it won only 3.4% of the vote and failed to gain any MEPs.
On Thursday they had to rebrand again, this time as The Independent Group for Change, because of a legal complaint from the petitions site change.org.
Mr Umunna said he was delighted to join the Lib Dems but has attacked the party on several occasions in the past.
In the run up to the 2017 general election, he said in a column in the Independent: The Liberal Democrats are trying to bury their recent past as the enablers of Tory austerity, but working people in this country will not forget or forgive the damage they did in government and what it is still doing to our communities today.
And he hit out at Mr Cable when his new leader was business secretary in 2013, tweeting: Vince Cable talks about increasing the minimum wage, but you can’t trust a word the Lib Dems say.