Three Conservative MPs have resigned, saying the party's "failure" to stand up to hardline Brexiteers has forced them to take the decision.
Sarah Wollaston, Heidi Allen and Anna Soubry have joined the new Independent Group.
In a joint letter to Prime Minister Theresa May, the MPs said: "We no longer feel we can remain in the party of a Government whose policies and priorities are so firmly in the grip of the ERG and DUP."
It comes hours after Labour MP Joan Ryan became the eighth MP from the party to resign and join the Independent Group.
The move takes the number of Independent Group MPs to 11.

Omani court sentences 2 foreigners to one month in prison for disturbing turtles
US Coast Guard suspends search for people on crashed medical aircraft
Brazil's Flavio Bolsonaro beats polls in presidential vote, faces Lula in runoff
Merz unveils fresh military aid in Kyiv, says Germany won't be intimidated
Protests demanding Indian election chief resigns escalate
