Three MPs - including Anna Soubry - have quit the Conservative Party to join the new Independent Group (TIG) in a major blow to Theresa May's authority.
The Broxtowe MP, along with Heidi Allen and Sarah Wollaston, hit out at the Prime Minister's "disastrous" handling of Brexit as they quit.
The three - who will join the eight ex-Labour MPs who have formed TIG - said: "The final straw for us has been this Government's disastrous handling of Brexit.
"Following the EU referendum of 2016, no genuine effort was made to build a cross party, let alone a national consensus to deliver Brexit.
"Instead of seeking to heal the divisions or to tackle the underlying causes of Brexit, the priority was to draw up 'red lines'. The 48% were not only sidelined, they were alienated."
They added: "Our politics needs urgent and radical reform and we are determined to play our part."
On joining The Independent Group, they added: "We intend to sit as independents alongside the Independent Group of MPs in the centre ground of British politics.
"There will be times when we will support the Government, for example, on measures to strengthen our economy, security and improve our public services.
"But we now feel honour bound to put our constituents' and country's interests first.
"We will continue to work constructively, locally and nationally, on behalf of our constituents."
Former education secretary and Conservative MP Nicky Morgan said she was "very sorry" to see her "principled and fearless friend" Ms Soubry resign.