The British Labour Party's shadow Commons leader Chris Bryant has resigned from Jeremy Corbyn's senior team.
Mr Bryant said: “We need someone new to unite and lead Labour.”
Eleven of Labour’s top team to quit on Sunday as a part of a protest against Mr Corbyn’s leadership.
A 12th, Hilary Benn, was sacked overnight after telling Mr Corbyn he had lost confidence in his ability to lead the party.
On a day of fast-moving events, shadow education secretary Lucy Powell, shadow health secretary Heidi Alexander, shadow minister for young people Gloria De Piero, shadow environment secretary Kerry McCarthy, shadow transport secretary Lilian Greenwood, shadow Scottish secretary Ian Murray, shadow Treasury chief secretary Seema Malhotra, shadow attorney general Karl Turner and justice policy spokesman Charles Falconer all resigned. Shadow Northern Ireland secretary Vernon Coaker has also resigned.
Several said in statements that they doubted Mr Corbyn’s ability to lead the party to victory in the country’s next parliamentary election.
Mr Corbyn’s allies insisted he would not bow to pressure to stand down, but rebels warned that if he tries to carry on he would be unable to form a new shadow ministerial team.
The mass walkout was triggered by Mr Benn’s sacking amid reports that he was working to co-ordinate a coup against the Labour leader.
Shadow chancellor John McDonnell, one of Mr Corbyn's closest allies, insisted the party leader is determined to carry on. But Ms Alexander said there are "a fair number" of other senior Labour figures considering their positions.
The resignations came as MPs prepared to discuss a motion of no confidence in Mr Corbyn – tabled by the veteran backbencher Dame Margaret Hodge – at the weekly meeting of the Parliamentary Labour Party on Monday.
It is expected to be followed by a vote of MPs in a secret ballot the next day. Although the motion has no formal standing, rebels are hoping that if it is passed his position will become untenable.
In a letter to Labour MPs, Dame Margaret warned the party could face a disaster at the polls if - as many MPs expect – the Conservatives trigger a snap general election.
“If a general election is called later this year, which is a very real prospect, we believe that under Jeremy’s leadership we could be looking at political oblivion,” she said.
But Mr McDonnell insisted there is no mood for a change at the top of the party among the grassroots activists who swept Mr Corbyn to the leadership and who would decide any contest.
He acknowledged, however, that a challenge may be unavoidable and said he is ready to chair Mr Corbyn’s campaign committee, as he did in last year’s leadership election. “If there is a leadership election, I’m hoping that we can get that over with fairly quickly, and we can get back to business in September,” he told BBC News.
“If it is triggered fairly quickly... I think we should try and get it over with in two months at least, maybe three maximum, but come back at least in October and get Jeremy back in position so we can then go straight into what could be a general election campaign.”
Shadow international development secretary Diane Abbott – another of Mr Corbyn’s allies – angrily accused the rebels of plotting their move for months. “The truth about today’s coup attempt is that it has been long-planned,” she told BBC News.
“This has been planned for months and ordinary party members will not understand why Labour MPs want to set themselves at odds with the membership and, at this very difficult time, choose to play what are essentially Westminster games.”
Mr Benn, who has been at odds with Mr Corbyn since they took opposing sides in the Commons vote on military action in Syria, said the party needs "strong and effective leadership" in the wake of the referendum vote.
"We don't currently have that and there is also no confidence that we will be able to win a general election as long as Jeremy remains leader," he told BBC One's The Andrew Marr Show. "It is for each individual to make their own decision, I have made mine and I made my views clear to Jeremy.
He is a good and decent man but he is not a leader and that is the problem.” Leaked documents suggest Mr Corbyn’s team moved to delay and water down the Labour Remain campaign, the BBC reports.
Reporting: Reuters/PA/Guardian