MP Heidi Allen, who quit the Conservative Party earlier this year, has joined the Liberal Democrats.
The MP for South Cambridgeshire left the Conservatives in February over its Brexit policy and other issues.
She subsequently became the leader of the fledgling Change UK but left after the party’s failure to win any seats in the European elections.
She is the fourth ex-Tory to join the Lib Dems in recent months, after Sarah Wollaston, Philip Lee and Sam Gyimah.
In a statement, she said she would fight the South Cambridgeshire seat for her new party at the next election.
Ms Allen said her former party and Labour had “moved to the extremes” and it was only the Liberal Democrats which now occupied the “liberal centre ground” of British politics.
‘Part of a team’
The MP, who has been sitting as an independent in Parliament for several months, said she realised she could be “stronger and more effective” as “part of a team”.
“Now is the time to stand shoulder to shoulder with, not just alongside, those I have collaborated and found shared values with,” she said.
Welcoming the party’s latest new recruit, the Lib Dem leader Jo Swinson said Ms Allen had “long been an ally” in the fight to stop Brexit.
She said her arrival showed that the Lib Dems were the “strongest party of Remain”.