Former Conservative MP Heidi Allen has joined the Liberal Democrats, the party has said.
The South Cambridgeshire MP is the seventh MP to join the party this year, taking its total in the Commons to 19.
She had been sitting as an independent since quitting the Change UK group in June.
Former Tory colleagues Sarah Wollaston, Sam Gyimah and Phillip Lee have joined the Lib Dems in recent months, as well as former Labour MPs Chuka Umunna, Angela Smith and Luciana Berger.
Ms Allen, who will stand in her seat as the party’s candidate at the next election, said: “Beyond a shadow of a doubt, I know that the best way to serve my constituents and country is to join the Liberal Democrats.
“Now is the time to stand shoulder to shoulder with, not just alongside, those I have collaborated and found shared values with.
“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 under the leadership of Jo Swinson.”
Ms Allen left the Conservatives in February, along with Ms 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 reported disagreements over strategy.
Her constituency is traditionally a solidly Tory seat, with Ms Allen having a majority of almost 16,000.
Lib Dem leader Jo Swinson said she was “delighted” to welcome Ms Allen to the party.
“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,” she said.
“This once again proves that the Liberal Democrats are the strongest party of Remain.
“The best deal we have is as members of the European Union, and I look forward to working with Heidi in order to stop Brexit.”
The party recently adopted a shift in Brexit policy, promising that a Lib Dem government will halt Brexit without a second referendum.
They had previously supported the revocation of Article 50 – to stop the UK’s departure from the EU – following a further public vote on EU membership.