Sam Gyimah – a candidate in this year's Tory leadership race – was unveiled as the unequivocally anti-Brexit party's newest MP at the Lib Dem conference opening rally on Saturday evening.
In a blistering attack on the prime minister, the ex-universities minister pledged to fight against Downing Street's "scorched earth approach" to Brexit and Mr Johnson's "nationalism".
Mr Gyimah was one of 21 Tory rebels to lose the whip for voting against the government on a crucial vote giving MPs control of the Commons order paper. He was still a member of the Conservative Party.
But in a major coup for new leader Jo Swinson, he now joins Dr Phillip Lee, another former Tory, who defected to the Liberal Democrats and walked across the floor of the Commons to the opposition benches during Mr Johnson's first ever prime minister's questions two weeks' ago.
Responding to Mr Gyimah's decision to join the Lib Dems, the shadow Brexit secretary Sir Keir Starmer said: "Johnson & Cummings are destroying the Conservative Party."
Speaking on Sky News' Sophy Ridge on Sunday, the new Lib Dem MP said it had "became very clear" since the Tory leadership race this summer that there were not enough moderate MPs left in the party.
"It has done something unprecedented – it has withdrawn the whip from moderate MPs," the East Surrey MP said. "The stark reality is that I had to face up to the fact that the Tory party is in a different place.
"If I want to stand up for liberal values, then the Conservative Party is no longer the place to do that."
Mr Gyimah's former colleague, the Tory MP Huw Merriam, added: "Very sorry to see another bright talent take the decision to sit opposite, rather than alongside, us on the government benches. We're a broad church so I hope others will keep the faith."
Alistair Burt, who was also stripped of the party whip, said: "The Conservative Party, my party, should read this with both shame and foreboding.
"To lose this sort of talent is not smart. Sam is not alone in that 40-50yr group ministerial quality wanting to stay or go. I want them to stay. Does the Conservative Party?"
But another Conservative MP, Crispin Blunt, said he felt "betrayed" by his decision to join the Lib Dems, claiming: "You had every opportunity to be a team player with Surrey and other neighbouring colleagues and it's clear you've only been in this business for yourself.
"If I believed this was about a principled belief in the European ideal I would be minded to understand. In your case this is about a principled belief in Sam G. At least I'm relieved of defending your corner any longer."
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