Former Conservative MP Sam Gyimah has joined the Liberal Democrats.
Six MPs have defected to the party in recent weeks, including former Tory MP Philip Lee, and ex-Labour MPs Luciana Berger and Chuka Umunna.
Mr Gyimah was one of the 21 Tories who had the Conservative whip removed after rebelling against Boris Johnson in a bid to prevent a no-deal Brexit.
Last December, the East Surrey MP quit as science and universities minister in a row over Theresa May's Brexit deal.
The 43-year-old briefly stood in the race to become Conservative Party leader after Mrs May quit.
The Lib Dems currently have 18 MPs, having been boosted by a victory in the Brecon and Radnorshire by-election and the defections.
Mr Gyimah told BBC News that "the hard Brexiteers have won in the Conservative party - it is a hard Brexit party".
"There just aren't enough Conservatives like me," he said, explaining his decision to join the Liberal Democrats.
"If I want to fight for the values for which I came into politics.. the values of tolerance, the values of being sensible and pragmatic and acting in the interest of the country, then the Liberal Democrats is where I can do that from."
Not long ago at Westminster, if you were on the hunt for a smile, you wouldn't bother with the Lib Dems.
There weren't many of them, for a start, and those left were the last survivors of a near apocalypse for the party; shrivelled, ignored and drowned out.
Not any more.
They are bouncy, tiggerish and expanding.
They hope their clarity on Brexit - win an election and scrap it - will win favour with Remain inclined voters who may find Labour's pitch rather more ambiguous.
But their newbies face a big challenge: can they, realistically, win the seats they currently hold as Liberal Democrats?
Or will they go hunting for more fertile Lib Dem territory elsewhere - potentially dislodging long standing local party stalwarts?
Addressing the Liberal Democrats conference in Bournemouth, Mr Gyimah said: "There is now no orderly way for the UK to leave the EU on October 31.
"If the prime minister got a deal at the European Council on October 17 and 18, it would not be possible for us to leave on October 31 in an orderly way."
He added that the government has been left in a position where "no-deal" is the only outcome that can be delivered.
He said he had been "disheartened" by the way the whipping process "had been framed... for us MPs to choose our careers, in other words our own salaries, over putting the country first."
Mr Gyimah, who has been sitting as an Independent after losing the Conservative whip, has been a prominent advocate for a second referendum.
He previously signalled his intention to stand as an independent candidate in East Surrey in the event of a snap general election.
Who is Sam Gyimah?
Mr Gyimah was born in Beaconsfield, in Buckinghamshire.
When he was six years old, his parents split up and he moved with his mother to her native Ghana, while his father remained in the UK.
He attended Achimota school, a state school in the capital of Accra, before returning to the UK to complete his GCSEs and A-levels at Freman College, a comprehensive in Hertfordshire.
Mr Gyimah went on to win a place at Somerville College, Oxford, to read philosophy, politics and economics (PPE), and served as president of the Oxford Union in 1997.
An Arsenal fan, he worked for Goldman Sachs for five years as an investment banker before moving into politics, standing unsuccessfully for Camden council elections in 2006.
In 2010 he became the MP for East Surrey and had been in Westminster for two years when he was made parliamentary private secretary to the then PM David Cameron.
He went on to become a government whip in 2013 and childcare and education minister a year later, before becoming prisons minister in 2016 and universities minister after that.
The married father-of-two quit as universities minister in December last year over Theresa May's Brexit deal.
He was introduced to delegates at the conference by the party's leader Jo Swinson as the "newest Liberal Democrat MP".
Speaking to the conference, Mr Gyimah said he did not take the decision to join the Lib Dems lightly and had started reconsidering his position in the Tories while Mrs May negotiated her deal with the EU.
But he said his concerns with the Conservative party now "go beyond Brexit".
"The values we have taken for granted for so long in our country... are under threat," Mr Gyimah said. "What Jo and I discussed are the Liberal Democrats have a unique opportunity to fight to defend those values and create a new force in British politics. That is why I find myself here today."
He said "the problem is not just on the Conservative side. When I look across the aisle, I also see on the Labour benches the same issue I have seen on the Conservative side, a doctrinaire, intolerant approach which means centrists are being squeezed out".
Mr Gyimah's move was welcomed by Lib Dem MPs.
Mr Umunna tweeted he was "absolutely delighted" and Layla Moran said: "Welcome... So delighted to have you on the team".
BREAKING: absolutely delighted to see @SamGyimah join the @LibDems today - just unveiled by @joswinson at #LibDemConf just now. He is a superb addition to the UK’s biggest and strongest #Remain party! pic.twitter.com/rM2TyY7EO6— Chuka Umunna (@ChukaUmunna) September 14, 2019
Chris White, a former government adviser, told the BBC it was "extremely disappointing" to see Mr Gyimah join the Lib Dems because he "stood on a manifesto pledge to deliver the referendum and here he is switching to a party which is manifestly not going to do that".
A bid by Mr Johnson for an autumn general election has so far been rejected by MPs who wanted to first make sure a bill designed to avoid a no-deal Brexit became law.
But since the bill, which seeks to force Mr Johnson to ask for a extension to the deadline, has been given Royal Assent, opposition MPs are preparing to start their general election campaigns.
As the Lib Dem conference opened, Ms Swinson said the party's anti-Brexit message should be "unequivocal" in a general election campaign.
She expressed her hopes that members would back her policy proposal of scrapping Brexit without another referendum.