MP Antoinette Sandbach, who was expelled from the parliamentary Conservative Party last month, has joined the Liberal Democrats.
The Remain-voting Cheshire MP was among 21 rebels who lost the Tory whip after rebelling against Boris Johnson in a bid to prevent a no-deal Brexit.
She will stand as a Liberal Democrat candidate in her Eddisbury constituency in December’s general election.
Explaining her decision, she said the Tory Party had “moved their values”.
Her move makes her the eighth MP to have joined the Lib Dems this year.
Speaking to Radio 4’s Today programme, she said she had considered not standing for re-election.
“Like many of the MPs that have stood down, I have been subjected to abuse.”
“It has been incredibly difficult for my family and for me. But this is a critical time in our nation’s history,” she said.
Announcing her decision earlier, as campaigning got under way ahead of the 12 December election, Ms Sandbach said: “People have a very clear choice.
“The Conservative Party offers years of uncertainty, whilst the Liberal Democrats will stop Brexit.
“I will stand on my strong local record, helping to secure local investment, fighting for fair funding for our schools and to secure additional funding in local health services.
“Our country deserves so much better than Boris Johnson and Jeremy Corbyn.”
Ms Sandbach was not among the 10 rebels readmitted to the party last month, shortly before the Commons backed the legislation to approve the 12 December election.
Earlier this month, she lost a confidence vote among her local party members – she described it as “symbolic” but added that “it most likely means that I am not going to be the Conservative candidate in the next election”.
Contesting her Eddisbury seat as a Conservative candidate in 2017, Ms Sandbach won a near-12,000 majority over Labour, with the Lib Dem candidate third with 2,804 votes.
She was among 19 former Tories who backed the prime minister’s Brexit deal legislation last week but voted against his proposed three-day timetable for it to be considered in the Commons ahead of the original Brexit deadline of 31 October.
Speaking after joining the Lib Dems, she said she was concerned Mr Johnson’s deal was “a trap door to a no-deal Brexit”.
She follows MPs Sarah Wollaston, Philip Lee and Sam Gyimah to become the fifth ex-Tory to join the Lib Dems in recent months.
Ex-Conservative Heidi Allen also joined the party earlier this month, after quitting the fledgling Change UK party she joined after leaving the Tories.
Liberal Democrat leader Jo Swinson said Ms Sandbach was a “passionate campaigner” and would be a “fantastic candidate” in the election.
“Her defection clearly shows that the Liberal Democrats are the strongest party of Remain and attracting support from right across the political spectrum,” she added.