The three MPs - Anna Soubry, Heidi Allen and Sarah Wollaston - said they would continue to serve in parliament alongside former Labour MPs who have recently quit the major opposition party.
Three MPs quit Britain's governing Conservative Party on Wednesday in protest at its "disastrous" hard line on Brexit, joining a rebellion in parliament that is redrawing the country's political landscape just weeks before it leaves the EU.
The resignations by the pro-European trio posed a fresh and embarrassing headache for Prime Minister Theresa May as she headed to Brussels for talks with European Commission President Jean-Claude Juncker.
Anna Soubry, Heidi Allen and Sarah Wollaston said they planned to sit in parliament alongside eight former Labour MPs who, also citing their opposition to Brexit, resigned from the main opposition party this week to form the new Independent Group.
"I'm not leaving the Conservative Party; it's left us," Soubry said at a press conference flanked by her colleagues following the announcement of their decision, arguing its "anti-EU wing" was now setting the agenda.
Their move has reinforced the view that Britain was plunging further into political turmoil, without an EU divorce deal just 37 days before it is due to leave the bloc on March 29.
The shake-up means that the Conservatives now have a working majority of just eight, thanks only to their confidence and supply arrangement for support from Northern Ireland's 10 Democratic Unionist (DUP) lawmakers.
The political impasse risks the country crashing out of the EU without an agreement, with the rising uncertainty blamed for a string of car-makers and other businesses recently announcing job cuts and reduced investment in Britain.
In damning indictments on May's leadership, the three Conservatives said in their letter to May that Brexit had "redefined" their party and was "undoing all the efforts to modernise it".
"The party that was once the most trusted on the economy and on business is now marching us towards the cliff edge of a no-deal Brexit," Wollaston said.
"None of us are prepared to wait until our toes are at the cliff edge before we take a stand," she added, urging Conservative ministers unwilling to defect to "have the courage of their convictions" and step down to oppose such a scenario.
May said in a statement she was "saddened" by the resignations and thanked the MPs for their "dedicated service to our party over many years".
She noted EU membership has been "a source of disagreement both in our party and our country for a long time" but the MPs' move would not stop Brexit.
"Under my leadership, the Conservative Party will always offer the decent, moderate and patriotic politics that the people of this country deserve."
Ms met with European Commission President Jean-Claude Juncker on Wednesday, hailing "progress" from her "constructive" meeting.
"So we’ve made progress," she said, telling reporters both sides had agreed to work on assurances over the "Irish backstop" issue.
With only 39 days until Britain leaves the EU, divisions over Brexit have fragmented British politics, breaking down traditional party lines and creating new ad hoc coalitions.