Chuka Umunna made a “serious mistake” in leaving Change UK, the party’s new leader Anna Soubry has said.
Six Change MPs – Heidi Allen, Sarah Wollaston, Angela Smith, Luciana Berger, Gavin Shuker and Mr Umunna – left the party last week.
But Ms Soubry said it was the MP for Streatham’s departure that would be most keenly felt.
“I will always be more sad than you can imagine that Chuka is not with us,” she told the Guardian.
“I think he’s a man of huge ability and talents, and I think he has made a very serious mistake.”
She added: “I said to him: ‘The movement’s out there, we just need to build a home for that movement, a political party.’
“When he decided that he believed we should not stand candidates I was beyond even disappointed.
“I believed in him, and believed he should be prime minister of our country. He was a major part of why I left the Conservative Party.”
Along with former Tory minister Ms Soubry, the remaining Change UK MPs are Chris Leslie, Joan Ryan, Mike Gapes and Ann Coffey.
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.