Three pro-European MPs quit the ruling Conservative Party to join a new centrist group of independents who oppose the government's determination to take Britain out of the European Union with or without a divorce deal.
Anna Soubry, Heidi Allen and Sarah Wollaston resigned to join eight ex-opposition British Labour Party MPs in an alliance dubbed the Independent Group.
The defections involve only a small fraction of the 650 politicians in the House of Commons, but mark the biggest shake-up in decades for Britain's political parties.
The breakaway MPs hope to gain members from disgruntled pro-Europeans in both the Labour and Conservative parties and forge a new force at the centre of British politics.
They are inspired in part by the "En Marche" movement of French president Emmanuel Macron, which dominated France's most recent presidential and legislative elections at the expense of the country's mainstream parties.
"Both our parties are broken. We are going to #ChangePolitics for the better," the group tweeted yesterday.
In a letter to prime minister Theresa May, the three ex-Conservatives accused party leaders of abandoning the political centre, and said "the final straw for us has been this government's disastrous handling of Brexit".
Britain's departure from the EU is scheduled for March 29, but no deal on divorce terms has been agreed on yet by British MPs.
The three departing politicians accused Mrs May's government of allowing the Conservatives' hardcore pro-Brexit wing to push the country to the edge of an economically damaging "no-deal" exit from the bloc.
Ms Allen said Mrs May had been "bullied into submission" by Brexiteers and was "dragging the country and Parliament kicking and screaming to the edge of a no-deal abyss".
Ms Soubry said attempts to modernise the right-of-centre party by encouraging a more diverse membership and tackling social problems had been wiped out by the all-consuming obsession with Brexit.
"I'm not leaving the Conservative Party - it has left us," she said.
Mrs May said she was saddened by the decision, but said the government was "doing the right thing for our country" by implementing voters' decision to leave the EU.
The three join eight British Labour rebels who quit the main opposition party this week over its direction under left-wing leader Jeremy Corbyn.
The Labour defectors accuse him of mounting a weak opposition to Mrs May's plans for leaving the EU and of failing to stamp out anti-Semitism in the party.
Joan Ryan, one of the ex-Labour MPs, said the party had become "infected with the scourge of anti-Jewish racism" under Mr Corbyn, a long-time supporter of the Palestinians.
With the clock counting down to Brexit and no deal in sight, members of the Independent Group want to hold a new referendum on the issue that could keep Britain in the 28-nation bloc.