Former Conservative MP Heidi Allen has joined the Liberal Democrats, the party said.
South Cambridgeshire MP is the seventh MP to join the party this year, bringing his Commons total to 19.
She has been independent since leaving Change UK in June.
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Former Tory colleagues Sarah Wollaston, Sam Gyimah and Phillip Lee have joined Lib Dems in recent months, as have former Labor MPs Chuka Umunna, Angela Smith and Luciana Berger.
Allen, who will remain in his post as party candidate in the next election, said: "Beyond a shadow of doubt, I know that the best way to serve my constituents and my country is to join the Liberal Democrats.
"Now is the time to be side by side with those I collaborated with and found shared values.
"To the extreme, conservatives and laborers have turned their backs on the liberal and progressive center for which the country is clamoring.
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"While facing the monumental task before stopping a damaging Brexit, healing the breaches in society and rebuilding the UK, there is only one party with the honesty, energy and vision to do so – and it is the Liberal Democrats under Jo Swinson's leadership. "
Allen left the Conservatives in February, along with Wollaston and Anna Soubry, to join the Independent Group, later renamed Change UK.
She became interim leader the following month, but left three months later amid disagreements over strategy.
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Her constituency is traditionally a solidly conservative chair, with Mrs. Allen having a majority of almost 16,000.
Lib Dem leader Jo Swinson said she was "delighted" to welcome Mme. Allen at the party.
"She has 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," she said.
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"This proves once again that Liberal Democrats are Remain's strongest party.
"The best deal we have is as members of the European Union, and I look forward to working with Heidi to stop Brexit."
The party has recently adopted a change in Brexit policy, promising a Lib Dem government to disrupt Brexit without a second referendum.
They had already supported the repeal of Article 50 – to prevent the UK from leaving the EU – after a new public vote on EU membership.
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