Labour and the Conservatives could face more resignations, with members of the new Independent Group saying they expect more MPs to join them.
Ex-Tory MP Heidi Allen told ITV's Peston programme "a third" of Tory MPs were fed up with the party's direction.
Labour MP Ian Austin told the Express and Star he would think "long and hard" about his future in the party.
It comes as Labour leader Jeremy Corbyn said defecting MPs should "resign and put themselves up for election".
Meanwhile, former attorney general Dominic Grieve said he would quit the Conservatives if there was a no-deal Brexit.
The Tory MP told the BBC's Newsnight he admired the courage of the 11 members of the Independent Group - eight ex-Labour MPs and three former Tory MPs - and agreed totally with their support for another EU referendum.
He said he would not be able to stay in the Conservative Party if it "went completely off the rails" and backed leaving the EU without a negotiated agreement.