Jeremy Corbyn's protracted reshuffle will continue as he seeks to fill holes left by three shadow ministers who quit in protest on a day of bitter recriminations within the party ranks at Westminster.
Kevan Jones, Jonathan Reynolds and Stephen Doughty all walked out citing differences with the Labour leader on key policy issues and complaining of "lies" and smears against sacked colleagues.
Shadow chancellor John McDonnell dismissed the trio as being from "a narrow right-wing clique" who refused to "respect" the result of the leadership election and accept Mr Corbyn's strong mandate from party members and supporters.
The close ally of the leader defended the replacement of the anti-Trident Maria Eagle as shadow defence secretary with Emily Thornberry, who shares Mr Corbyn's desire to scrap the nuclear deterrent.
But Lord Mandelson was among a string of senior figures to warn that shifting the Opposition back to the policy of unilateral disarmament ditched in the 1980s would leave it "even further away from any prospect of winning a general election".
Defence Secretary Michael Fallon used the chaos - on the day North Korea claimed to have exploded its first hydrogen bomb - to claim that Labour was a threat to national security.
Mr Jones - who was a member of the defence team - said the appointment of Ms Thornberry was "a mistake" though his claim that the view was shared by Tom Watson was denied by the deputy leader.
The North Durham MP also joined calls for her to return donations from a law firm facing disciplinary action over its role in an official inquiry into allegations of murder and torture by the British military.
Prime Minister David Cameron said there were "serious questions to answer" about her financial links to Leigh Day,
The sackings of Michael Dugher as shadow culture secretary - a role handed to Ms Eagle - and shadow Europe minister Pat McFadden for alleged "disloyalty" by criticising the leadership, sparked anger from moderate MPs.
John Woodcock said he was "losing hope" that the leadership was able - or even willing - to make sure the party stood a chance of winning the 2020 general election.
The sackings represented "pretty disgraceful conduct from the leader of a party that seeks to govern", he added.
Ms Thornberry insisted she wanted a "truly open policy review" on the future of Trident - but Mr Corbyn hopes to change policy-making rules to allow it to be set via a poll of members and supporters.
The former human rights barrister defended Leigh Day - which funded a legal research assistant for her office during her time as shadow attorney general to the tune of £14,500 - as an "outstanding firm".
It has been referred to the Solicitors Disciplinary Tribunal over allegations - which it denies - that it destroyed a key document at the centre of an inquiry, which eventually found claims of murder and torture by British soldiers were "completely baseless".
Lord Mandelson said the party was "too far behind, too far out of touch and too wide of the electoral mainstream to catch up and stand any realistic chance of replacing the Conservatives, however unpopular they will be in four years' time."
Mr Reynolds tore into shadow international development secretary Diane Abbott after she wrongly dismissed those who had quit as career politicians who had all been special advisers.
Ms Abbott said: "If you look at Jonathan Reynolds, if you look at Mr Dugher, if you look at some of these others, what do they have in common? They are all former special advisers."
A furious Mr Reynolds hit back: "At least Google us before slagging us off.
"For the record ... I was a trainee solicitor when elected, having gone to law school as a mature student and single parent," he said.
"And I think you're a total sell-out for sending your own kids to private school."
Mr Doughty dismissed her attack as "nonsense", pointing out he worked for more than seven years for international charities including Oxfam and campaigns such as Make Poverty History before an 18-month spell as a special adviser at the Department for International Development under the last Labour government.