Former Labour and Change UK MP Chuka Umunna has joined the Liberal Democrats, saying he was "wrong" to think "millions of politically homeless people... wanted a new party".
The Streatham MP said he had "massively underestimated just how difficult it is to set up a fully fledged new party without an existing infrastructure".
He was one of six MPS to quit Change UK - founded in February - last week.
It gained only 3.4% of the vote in the European elections.
In contrast, the Liberal Democrats - who, like Change UK, campaigned on a strongly pro-EU message - saw a surge in support, coming second after Nigel Farage's Brexit Party.
Mr Umunna's move to the the Liberal Democrats brings the party's number of MPs to 12.
Asked if he would hold a by-election and re-stand as a Lib Dem, Mr Umunna declined to answer directly but said that he had listened to his constituents and their biggest issue was Brexit.
The former shadow business secretary, who had previously criticised the Lib Dems for "enabling Tory austerity" during the 2010 to 2015 coalition government, acknowledged that not everyone in the party would welcome his arrival.
However, he told the BBC "things have changed", as the Lib Dems had voted against every single Conservative Budget since 2015 and had stood on an anti-austerity manifesto in the 2017 general election.
"If you want to end austerity you cannot do that if you are going to sponsor Brexit in the way that the two main parties are doing," he added.
Mr Umunna said he had realised "there isn't room for more than one centre-ground option" in British politics, adding that he believed there were "a good handful" of Conservative and Labour MPs who knew their parties were "broken" and could also be prepared to join the Lib Dems.
The MP, who withdrew from the 2015 Labour leadership contest days after announcing his candidacy, told the Times he did not want to take sides between the two contenders to replace Sir Vince Cable as Lib Dem leader, Jo Swinson and Ed Davey, adding: "I'm a newbie."
Welcoming him, Sir Vince 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."
When asked if he expected other MPs to defect to his party, the Lib Dem leader confirmed he was "in conversations" with other independent MPs.
Mr Umunna's move was also welcomed by the two candidates for the Lib Dem leadership.
Jo Swinson said the Lib Dems were "the rallying point for people who want to stop Brexit and fight the climate crisis", while Ed Davey praised the Streatham MP's "huge courage".
Change UK - formerly known as The Independent Group - was formed by MPs who quit Labour and then joined by some former 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.
After last month's European Parliament elections, six of its 11 MPs quit. On Thursday it applied to change its name to The Independent Group for Change, to avoid a protracted legal dispute with petitions website Change.org.
Labour Party chairman Ian Lavery called for a by-election in Mr Umunna'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."