Former Tory minister Sam Gyimah is latest to defect to Liberal Democrats
He has been a prominent advocate of a second referendum and was briefly in the race to replace Theresa May as Conservative leader in June.
Announcing his decision to join the Lib Dems on the first day of their annual conference in Bournemouth, the former education minister said centrists were being "cast out of both main parties".
He told the Observer: "I listen to ministers undermining the courts ... Ministers questioning experts because their views are inconvenient for what the Government is saying about no-deal.
"You have a government that says law enforcement is the centrepiece of its platform and yet says in another breath that it will pick and choose what laws it chooses to respect.
"This is in many ways undermining key pillars of our constitution and the functioning of our democracy. The issue for me is not just Brexit. It is beyond Brexit - how you conduct politics and the veering towards populism and English nationalism."