Eddisbury MP Antoinette Sandbach has defected to the Liberal Democrats.
Formerly a Conservative MP, Ms Sandbach lost the party whip when she defied Boris Johnson and voted against the government, and blocking a no-deal Brexit last month.
She becomes the fifth former Tory to join the Lib Dems in recent months.
It means that the party (which had 12 MPs following the 2017 election) will have its ranks swollen to 20 when the House of Commons officially breaks up next week.
Antoinette will fight to retain the Eddisbury seat, where the former Lib Dem candidate Ian Priestner came third – securing less than 3,000 votes.
Ms Sandbach told The Times newspaper that she felt “excited and almost liberated” to be joining the Liberal Democrats.
The defection had been discussed for months, she explained, and was not a direct result of her losing the Tory whip.
She said: “The fact the government seemed to be rushing headlong into no deal was effectively the final push.”
Ms Sandbach added: “I’m standing as a Liberal Democrat because of the values of the party.
“I could have chosen to stand as an independent… I want to stand for a progressive centre-ground liberal party.
“And I see the two main parties being stuck in the past and floating off to two extremes. And the Liberal Democrats reflect my values.”
The defection will come as a welcome boost to party leader Jo Swinson on the first proper day of campaigning for the December general election.
Antoinette admitted she had not fully digested the full details of all Lib Dems’ policies.
Ms Sandbach backed Remain in 2016 but voted for Theresa May’s Brexit deal three times.
She’s described in The Times’ article as being “comfortable” with her new party’s policy of revoking Article 50 in the event they win a majority.
Ms Sandbach, a former Barrister, first became an MP in 2015 having previously served in the National Assembly for Wales.
Despite her constituency backing leave in the EU referendum, she maintained her seat in 2017 with a majority of almost 12,000.