Michael Dugher has told BBC News he was sacked by Jeremy Corbyn for speaking up for colleagues who had been "trashed" by members of the Labour leader's team.
The ex-shadow culture secretary was the first casualty of Mr Corbyn's front bench reshuffle.
Shadow defence secretary Maria Eagle has been told that she will be moved. And Pat McFadden is to be replaced by Pat Glass as shadow Europe minister.
Mr Corbyn has been holding meetings with close aides throughout the day.
A planned meeting of the shadow cabinet at lunchtime was cancelled as the changes were taking longer than planned.
Mr Dugher - who ran Andy Burnham's leadership campaign - told BBC News: "I decided to speak out a number of days ago because what we've seen in recent weeks is a number of good hardworking loyal members of the shadow cabinet being systematically trashed, in terms of their reputations, in their newspapers by people in the employment of Jeremy Corbyn.
"It was de-stabilising and I felt it was right to speak out about that and I appear to have paid the price for that. But I don't regret it for one second."
He also said that the biggest casualty in the reshuffle had been the "new politics" and despite promises from the leader that there would be room for a little dissent, "the truth is that's just not transpired".
The sacking of the Barnsley East MP, a close ally of former leader Gordon Brown who is known for his combative style, has been met with dismay by senior Labour figures.
Shadow defence minister Toby Perkins said: "Sad to hear this, always effective at putting Tories on the back foot, something we need to do more of."
Labour deputy leader Tom Watson said: "Michael Dugher is a rare politician - a talented working-class MP who hasn't lost his strong Yorkshire roots.
"Politicians with his ability and commitment can make a difference in any role. Labour's loss in the shadow cabinet will be compensated by Michael's free thought on the backbenches."
Analysis by BBC Political Editor Laura Kuenssberg
Will this turn out to be the reshuffle that never was?
One member of the shadow cabinet said, "it was all on, and then it was all off again".
A senior Labour figure tells me now that despite the target on Hilary Benn's back, that Jeremy Corbyn has "backed down", and the shadow foreign secretary is absolutely safe in his job.
Whatever the final details today, the bigger question in the longer term is whether even the threat of sackings will change the dynamics...
There has been speculation Mr Corbyn would purge his front bench team of critical voices so that the party could have a unified position on important issues such as defence and security.
Mr Corbyn has held lengthy talks with Mr Benn, who spoke out against Mr Corbyn's position on bombing Syria, and Maria Eagle, who opposes the Labour leader on nuclear weapons.
The BBC understood on Monday evening that Mr Benn would be keeping his job, but sources have since suggested that he may yet be moved, or sacked.
A further nine shadow cabinet members voted for air strikes after Mr Corbyn was forced to allow a free vote. There is speculation that shadow ministers have threatened a mass walk out if Mr Benn was sacked.
Ken Livingstone denied Mr Corbyn had "bottled" his reshuffle.
The former London mayor, an ally of Mr Corbyn, said he did not believe the Labour leader had ever intended to sack Mr Benn and speculation about his future had been whipped up by the media.
He also suggested Mr Benn's position had become more secure because he had "stopped being quite so critical and dissident".