Former Labour frontbencher Dan Jarvis has suggested he may not represent the party at the 2020 election if its policy is to scrap the Trident nuclear deterrent.
The Barnsley Central MP, an ex-soldier who has been tipped as a future leader, said he would feel "deeply uncomfortable" fighting on such a platform.
The intervention comes after a tumultuous week for Labour in which Jeremy Corbyn shifted pro-Trident Maria Eagle from shadow defence secretary and replaced her with Emily Thornberry, who supports his call for Britain to disarm unilaterally.
The move paves the way for Labour's policy review, which is now jointly chaired by Mrs Thornberry and veteran left-winger Ken Livingstone, to recommend a change in its official position.
The final decision would be taken by party conference, and some believe unions would block any effort to oppose Trident renewal for fear of job losses.
In an interview with The Guardian, Mr Jarvis echoed the concerns of many Labour moderates.
"To my core I have always been Labour and always will be, but I would feel deeply uncomfortable fighting as a Labour candidate on a manifesto that committed us to getting rid of our nuclear deterrent, not least because we would lose the election," he said.
"It's an issue of such strategic importance with the public that it would be catastrophic for us to go into an election with that as our policy."
Mr Jarvis suggested his decision not to contest the Labour leadership last year had been a mistake.
"I'm not a great one for regretting anything. But what I do regret is that I didn't give it more thought beforehand."
Michael Dugher, who was sacked as shadow culture secretary in the Labour reshuffle, warned that former London mayor Mr Livingstone was effectively "pulling the strings" of the party leadership.
"If you look at the people around Jeremy, most of them have been in the employment of Livingstone," he told The Times.
"I think the truth is this leadership team is dancing on the strings of Ken Livingstone - and nobody elected Ken Livingstone to be leader of the Labour Party last September."
Mr Dugher added that Mr Corbyn keeps "very bad company" and described the reshuffle, which took most of the week to complete, as an "unmitigated shambles".
Kevan Jones, one of three shadow ministers who quit in protest at Mr Corbyn's changes, said Mr Livingstone "seems to have free rein to say and insult whoever he likes without any real sanction".
Meanwhile, the BBC has rejected an official complaint from Labour that it "orchestrated" the resignation of frontbencher Stephen Doughty on live television.
The leader's communications director Seumas Milne branded Mr Doughty's dramatic announcement during the Daily Politics programme on Wednesday an "unacceptable breach" of the corporation's "role and statutory obligations".
"By the BBC's own account, BBC journalists and presenters proposed and secured the resignation of a shadow minister on air in the immediate run-up to Prime Minister's Questions, apparently to ensure maximum news and political impact," he said.
The BBC's editor of live political programmes Robbie Gibb replied: "I reject your suggestion that we orchestrated and stage-managed the resignation of Stephen Doughty.
"As he himself confirmed on Friday, Mr Doughty had decided to resign his frontbench position on Wednesday morning, before speaking to any journalists.
"Your letter suggests that our decision to interview Mr Doughty in the run-up to Prime Minister's Questions was designed to 'promote a particular political narrative'. This is simply not the case."
Mr Doughty has accused Mr Corbyn's senior aides of "smearing" him, and insisted he sent his resignation letter to Mr Corbyn before appearing on TV.