Angela Eagle has fired her opening salvo at Jeremy Corbyn in a Labour leadership battle which has the potential to split the party and could end up in a bitter legal wrangle.
The former shadow cabinet minister - the most senior MP to resign in the revolt against Mr Corbyn - said he is "not a leader" and accused him of "hiding behind a closed door" in denial of the chaos around him.
Mr Corbyn has vowed to resist the challenge, saying it would be "irresponsible" for him to quit, and threatened legal action if his name is not on the ballot in a leadership contest.
Owen Smith, another potential challenger, demanded emergency talks with Mr Corbyn and suggested the leader and allies are prepared to split the party.
Angela Eagle looked for journalists' questions at her Labour leadership bid; but they'd left to cover Conservatives https://t.co/o2GLUTP0qN— Sky News (@SkyNews) July 11, 2016
Speaking to ITV's Peston on Sunday, Ms Eagle - a former Northern Ireland Office minister - said Mr Corbyn had not been able to "communicate with the electorate" and "he's now lost the confidence of the parliamentary party".
She added: "Jeremy lost us local councillors in the elections, we have failed to win the EU referendum which is going to cause enormous stress and pressure in our country, that is not the leadership that will take us forward.
"I tried over nine months to support Jeremy and his leadership. He's not a bad man. He's not a leader, though."
Mr Corbyn and his allies insisted Labour rules mean he will automatically be on the ballot and any challenger must secure the names of 51 MPs - 20% of the party's parliamentarians in Westminster and Brussels - to be nominated.
But opponents have interpreted the document to mean that Mr Corbyn will also require the support of MPs to stand - which is unlikely to happen.
The party's National Executive Committee (NEC) will decide on the rules after a contest is formally triggered, with Ms Eagle to officially launch her bid today.
Ms Eagle did not set out policy areas where she differs from Mr Corbyn, but said he cannot win a general election.
She told BBC One's Sunday Politics it was "not clear from the Labour Party rules" if Mr Corbyn should be on the ballot, but added: "Anyone who aspires to lead the parliamentary party who cannot get 51 members, 20% of the parliamentary party, to back them is not going to be able to do the job properly."
On BBC One's Andrew Marr Show, Mr Corbyn said he had "reached out in a way no other leader has" in an attempt to unite all parts of the party."
He said he would be prepared to go to court if the NEC ruled he would not automatically be on the leadership ballot.
In a sign the party is on the verge of all-out civil war, Mr Smith said at a meeting with Mr Corbyn he asked him three times whether he was prepared to see a split but "he offered no answer", while Corbyn ally and shadow chancellor John McDonnell "shrugged and said 'if that's what it takes'".
Mr Smith said: "I am not prepared to stand by and see our party split. He said he asked to meet Mr Corbyn again today "to see how we can stop that".
Mr McDonnell said Mr Smith's claim was "complete rubbish", but former shadow cabinet minister Kate Green said: "I was in that meeting John. I heard you say it. Are you backtracking?"
Mr Corbyn told BBC Radio 4's The World This Weekend he was happy to meet Mr Smith and said: "I'm not splitting anything.
"I'm campaigning for a Labour government, I'm campaigning for social justice in this country. That's what unites people."
Rebel Labour MPs, including Ms Eagle, have been targeted on social media by activists supporting Mr Corbyn and the party leader was challenged about the abuse on the Marr Show.
Mr Corbyn said: "Nobody does vile abuse in my name with my approval, my support. I absolutely totally condemn it in every way, just as much as any abuse that's hurled at me or anybody else. It's simply wrong. I urge people to engage in political debate, not media abuse of any sort." Challenged about comments describing Ms Eagle as a "treacherous b****" and a "Tory-supporting weasel", Mr Corbyn said that was "totally unacceptable language".
Jeremy Corbyn addressed tens of thousands of supporters at the Durham Miners' Gala and vowed to return as prime minister. More than two decades after the last pit closed in the Durham coalfield the Miners Gala or the Big Meeting as it is known locally remains as popular as ever with over 150,000 expected to attend this year. The gala forms part of the culture and heritage of the area and represents the communal values of the North East of England. The gala sees traditional colliery bands march through the city ahead of their respective pit banners and pass the County Hotel building where union leaders, invited guests and dignitaries gather on the balcony before then heading to the racecourse area for a day of entertainment and political speeches. Beginning in 1871 the gala is now the biggest trade union event in Europe and many thousands of people continue to attend each year.