Six Change UK MPs have quit the party, including its outgoing leader.
The remaining supporters in parliament are Anna Soubry – who is the new leader – Chris Leslie, Joan Ryan, Mike Gapes and Ann Coffey.
Former Conservative MP Ms Soubry said she was “deeply disappointed” at the split.
“Now is not the time to walk away, but instead to roll up our sleeves and stand up for the sensible mainstream centre ground which is unrepresented in British politics today,” she added in a statement.
Her predecessor, fellow ex-Tory MP Heidi Allen, said the departing MPs would now “support each other as an independent grouping”.
She said she was “immensely proud” of Change UK candidates and supporters but that “our priority right now must be to provide collegiate leadership to bring people together in the national interest”.
Former Labour MPs Chuka Umunna, Angela Smith, Luciana Berger and Gavin Shuker have also left, along with a second ex-Conservative MP Sarah Wollaston.
Change UK formally became a political party in April, to fight in the European Parliament elections.
It grew out of The Independent Group of MPs who split from Labour and the Conservatives in February over Brexit and Jeremy Corbyn’s handling of antisemitism complaints.
Despite a slick launch in Westminster, the group was plagued 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 existing name because the Electoral Commission bans parties having the word “independent” in their name.
And in the election it had suffered all the stumbles for, it won only 3.4% of the vote and failed to gain any MEPs in Strasbourg.