Labour is braced for a bruising leadership contest with the potential to split the party after Angela Eagle said she would launch a bid to oust Jeremy Corbyn.
The two rivals will make televised pitches to supporters on Sunday morning in broadcast interviews after a dramatic series of developments ended the stand-off over the embattled leader's position.
Peace talks collapsed when deputy leader Tom Watson walked away from union-brokered negotiations aimed at ending weeks of deadlock as a defiant Mr Corbyn stayed in his post despite a revolt by the overwhelming majority of his MPs.
Shortly after Mr Watson's announcement, Ms Eagle said she would formally launch her leadership bid on Monday.
The leader of Britain's biggest trade union branded the decision by Mr Watson to pull the plug on negotiations an "act of sabotage" and warned it could lead to a schism in the party.
The crisis within the Labour ranks has seen scores of frontbenchers resign, with Ms Eagle the most senior member of the shadow cabinet to quit, and a motion of no confidence in Mr Corbyn by his MPs was backed by 172 votes to 40.
Former shadow business secretary Ms Eagle said Mr Corbyn had "failed to fulfil his first and foremost duty, that is to lead an organised and effective Parliamentary Labour Party (PLP) that can both hold the Government to account and demonstrate we are ready to form a government in the event of a general election".
Mr Watson said he had pulled out of the union talks because Mr Corbyn's intention to continue "come what may" meant "there is no realistic prospect of reaching a compromise" over his future.
The vote of no confidence in Mr Corbyn by the party's MPs showed he had "lost the support of the PLP with little prospect of regaining it", Mr Watson said.
But Len McCluskey, general secretary of Unite, said a workable plan to resolve bitter differences between Mr Corbyn and the PLP had "never been closer".
Mr McCluskey said Mr Watson's actions "look like an act of sabotage fraught with peril for the future of the Labour Party".
He stressed that Mr Corbyn's resignation had not been on the agenda for the talks and it was "deeply disingenuous" for Mr Watson to suggest that the leader's refusal to step down led to the collapse of the negotiations.
Mr Corbyn has insisted his name will automatically be on the ballot if a leadership challenge is launched against him and Mr McCluskey warned that if there was any legal attempt to prevent him fighting for his position it could risk a schism within the Labour movement.
Mr McCluskey said: "I must warn that any attempts to keep Jeremy Corbyn, elected just 10 months ago with an enormous mandate, off the ballot paper by legal means risks a lasting division in the party.
"It is time for everyone to commit to a democratic and dignified procedure as the only way to avert such a disaster for working people."
Dave Ward, Communication Workers Union general secretary, said: "Jeremy Corbyn retains the CWU's full support and it is clear to us that the actions of some members of the PLP, in undermining his mandate from the membership, are putting the future of the party at risk."
Former shadow work and pensions secretary Owen Smith, who is also thought to be considering a leadership bid, said he would meet Mr Corbyn "to explore any and all avenues to save our party" and would do "anything necessary" to prevent a split.
He said: "This is the greatest crisis facing Labour in generations and it comes at a time when our country is in desperate need of a united Labour Party to speak for Britain.
"I remain extremely concerned that a small number of people from both the left and right of our party seem intent on letting it split. The Labour movement must come together to avoid this at all costs.
"I remain committed to doing anything necessary to prevent a split and unite the party."