Former Labour and Change UK MP Chuka Umunna has joined the Lib Dems.
The Streatham MP told the Times he had been “wrong” to think “millions of politically homeless people… wanted a new option on the ballot paper”.
“I also think I vastly underestimated the importance of having an infrastructure and existing relationships with voters,” he added.
He was one of six Change UK MPs to quit last week, after it gained only 3.4% of the vote in the European elections.
The pro-EU Lib Dems saw a surge in support in the same elections, coming second after the Brexit Party.
Former shadow business secretary Mr Umunna told the Times that the Lib Dems’ success shows the potential for the party to be the “spearhead” of a “new progressive movement in this country”.
“I think we’ve got to grab the chance to fundamentally change the system forever now.”
Mr Umunna’s decision to become a Lib Dem takes the party’s total number of MPs to 12.
He previously criticised the Lib Dems for “enabling Tory austerity” while in coalition with them and acknowledged that not everyone in the party would welcome his arrival.
The MP, who withdrew from the 2015 Labour leadership contest days after announcing his candidacy, also ruled out standing for Lib Dem leader. He said he did not want to take sides between the two contenders to replace Sir Vince Cable, Jo Swinson and Ed Davey adding: “I’m a newbie.”
Welcoming him, Sir Vince Cable 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.”
Labour Party chairman Ian Lavery called for a by-election in Mr Ummuna’s constituency, 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.”
Change UK – formerly known as The Independent Group – was formed earlier this year by MPs who quit Labour and the Conservatives.
It pledged to push for any Brexit deal negotiated by the government to be voted on at a referendum – or “People’s Vote” – in which it would campaign for the UK to remain in the EU.
But in last month’s European Parliament elections, it gained only 3.4% of the vote and six of its 11 MPs quit. On Thursday it applied to change its name to The Independent Group for Change, after a dispute with petitions website Change.org.