Three Labour MPs have quit the party's front bench in protest at sackings made by Jeremy Corbyn in his reshuffle.
Jonathan Reynolds and Stephen Doughty quit over the sacking of the shadow Europe minister Pat McFadden.
Mr Corbyn fired Mr McFadden over "disloyalty" after he appeared to criticise his stance on terrorism.
Kevan Jones has quit his defence role over Trident after Mr Corbyn replaced pro-nuclear weapons MP Maria Eagle with unilateralist Emily Thornberry.
Shadow foreign secretary Hilary Benn - who also disagrees with Mr Corbyn on key issues including bombing IS targets in Syria - escaped the axe in the shake-up.
He denied he had been "muzzled" by Mr Corbyn after reportedly agreeing not to criticise the leader's policy positions from the front bench, saying he would be carrying on with his job "exactly as before".
The only changes in the shadow cabinet see anti-Trident MP Emily Thornberry replacing shadow defence secretary Maria Eagle, who moves to culture to replace sacked Michael Dugher.
But the moves have sparked a string of departures in the junior ranks.
In his resignation letter, Mr Reynolds backed comments by Mr McFadden in the aftermath of the Paris terror attacks, in which he criticised the response of the Stop the War coalition, which Mr Corbyn used to chair.
Mr McFadden said Mr Corbyn had told him he thought his comments were "an attack on him and that he had come to the conclusion because of that and one or two other things that I shouldn't continue".
"He clearly feels that me saying terrorists are entirely responsible for their action, that no-one forces anyone to kill innocent people in Paris, to blow up the London Underground, to behead innocent aid workers, that when I say they are entirely responsible for that, he clearly interpreted that as an attack on him," he added.
Shadow foreign minister Stephen Doughty - who announced his resignation on the BBC's Daily Politics - said Mr McFadden had been "singled out for punishment for speaking with honesty and principle".
Could Benn be a front and backbencher?
By Political Reporter Brian Wheeler
John McDonnell suggested in a BBC News Channel interview that Hilary Benn could emulate himself and Mr Corbyn, who spent their lengthy careers to this point speaking from the back benches, often against their party leader.
Not to be uncharitable to the pair, but hardly anyone in the media was interested when they did it. But if a senior figure like Mr Benn did it, it would almost certainly become the story. Conservative MPs would be beside themselves with glee.
Kevan Jones who last year clashed with Ken Livingstone, when the anti-nuclear former mayor was put in joint charge of Labour's defence review, said he "respected" Mr Corbyn's view that Britain's nuclear deterrent should be scrapped, but disagreed and could no longer remain as shadow defence minister.
He described Ms Thornberry's appointment as a mistake, saying it left the party's defence policy "controlled by the North London Labour party".
But Ms Thornberry, the MP for Islington South, defended her suitability for the role and promised that, while she had clear personal views on Trident, she would lead a "truly open review" on the UK's nuclear deterrent.
In an interview with the British Forces Broadcasting Service, she said she had "quite a lot more experience" than people were giving her credit for, pointing out she had family members in the military and a "regiment in my constituency".
Shadow Chancellor John McDonnell defended the decision to sack Mr McFadden, saying there had been other "issues about loyalty" and it was not just about one Commons statement.
He told the Today programme Mr McFadden's comments had "played into an agenda which distorted Jeremy's views on how we tackle terrorism" and this had contributed to the "undermining of Jeremy's status".
Later he told Channel 4 News there was a "hard right" faction in the party which had never accepted Mr Corbyn's election victory.
'Comedy of errors'
At Prime Minster's Questions, David Cameron said it "speaks volumes" about Mr Corbyn's leadership that Mr McFadden should be sacked for saying terrorists should be held responsible for their actions and he questioned why Mr Benn had not quit the front bench team.
He then poked fun at Labour through the medium of Shakespeare quotes.
Responding to a question from Stratford-on-Avon Conservative MP Nadhim Zahawi, he said: "There was a moment when it looked like this reshuffle could go into its Twelfth Night. It was a revenge reshuffle so it was going to be As You Like It.
"I think though we can conclude it's turned into something of a Comedy Of Errors, perhaps Much Ado About Nothing? There will be those who worry Love's Labour's Lost."
The Conservatives said Labour was now a "threat to national security", citing Ms Thornberry's views on Trident and comments Mr Corbyn has made in the past about Nato and the role of the UK armed forces.
Mr McFadden is being replaced by former junior shadow education minister, Pat Glass.
There are now 17 women and 14 men in the full shadow cabinet. Elsewhere, Emma Lewell-Buck was promoted to shadow minister for devolution and local government.
Responding to comments by Labour leadership sources that he was sacked for "incompetence and disloyalty", sacked shadow culture secretary Michael Dugher tweeted that he was "not sure it's sensible for the leader and his office to get into a debate about 'loyalty' or 'competence'".
Confirming his own dismissal earlier on Tuesday, Mr Dugher - a former aide to Gordon Brown - said he had "paid the price" for speaking out in defence of colleagues whose reputations he claimed had been "trashed" by aides to Mr Corbyn.