Former Conservative MP Heidi Allen has joined the Liberal Democrats, the party has said.
The South Cambridgeshire MP, who has been sitting as an independent since quitting the Change UK group in June, takes the Lib Dem tally in the Commons to 19.
Her move follows the recent arrivals of ex-Tories Sarah Wollaston, Sam Gyimah and Phillip Lee as well as former Labour MPs Chuka Umunna, Angela Smith and Luciana Berger.
Ms Allen, a committed Remainer, said she was joining the Lib Dems as the other main parties had turned their backs on the "liberal progressive centre ground".
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"Now is the time to stand shoulder to shoulder with, not just alongside, those I have collaborated and found shared values with," she said.
"Shifting to the extremes, the Conservatives and Labour have turned their backs on the liberal, progressive centre ground our country is crying out for.
"As we face the monumental task ahead of stopping a damaging Brexit, healing the rifts in society and rebuilding the UK, there is only one party with the honesty, energy and vision to do that - and that is the Liberal Democrats."
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Ms Allen's move was warmly welcomed by Lib Dem leader Jo Swinson.
"She has long been an ally in the fight to stop Brexit, and having worked with her I know the energy and passion she will bring to our party," Ms Swinson said.
"This once again proves that the Liberal Democrats are the strongest party of Remain."