Former London mayor Ken Livingstone will have no formal role in Labour's review of its defence policy, the party has announced.
The terms of reference for the review were published by shadow defence secretary Emily Thornberry, who said she aimed to produce an interim report by June for consideration by the party's national policy forum and annual conference.
The review is widely expected to bring tensions within the party over nuclear disarmament to a head by recommending a switch in policy away from the current position of support for the renewal of Britain's Trident deterrent, in line with leader Jeremy Corbyn's long-held unilateralist views.
Ms Thornberry said she was "extremely sceptical" about Trident and would ask some "very difficult questions" about its future.
The appointment in November of Mr Livingstone as co-convenor of a commission on international policy - which includes security and defence - was widely seen as an attempt to curb the influence of then-shadow defence secretary Maria Eagle on the decision over Trident renewal. Pro-renewal Ms Eagle was replaced in last week's reshuffle by Ms Thornberry.
The shadow defence secretary told the BBC: "My views are on the record: I am extremely sceptical about Trident and I will not be afraid to ask some very difficult questions and I need to hear the evidence about it and I will then come to a view.
"I go into this wanting to look at the evidence before we make policy."
As recently as Wednesday, Mr Livingstone - a close ally of the party leader - appeared to suggest that he expected to be working on the review, saying he hoped the recommendation on Trident could be rushed through within 8-10 weeks, which would be "a lot of work for me and Emily".
However, after Ms Thornberry launched the review alone, Mr Livingstone said that he had voluntarily decided to step aside following a discussion over lunch in which they agreed on all aspects of defence policy.
"I said: 'You and I agree on absolutely everything. There wasn't a single thing we didn't agree on. I think you should lead on this. You are the defence spokesperson, you have got to explain it in Parliament and at the next election.' I am delighted," he told Channel 4 News.
Labour sources confirmed that Mr Livingstone would not be acting as co-chair. Asked about Mr Livingstone's position, a Labour spokeswoman said: "There is no formal role in the defence review."
But she pointed out that anyone could make a submission to the review and it would feed in to the international policy commission, of which Mr Livingstone is co-convener with shadow foreign secretary Hilary Benn.
Any attempt to change policy on Trident is likely to provoke a split in Labour's frontbench team, with three shadow cabinet members - shadow work and pensions secretary Owen Smith, shadow education secretary Lucy Powell and shadow justice secretary Lord Falconer - already refusing to rule out quitting their posts if the party drops its backing for the nuclear deterrent.
Prime Minister David Cameron is expected to call a Commons vote on Trident renewal within the coming months.
Although the review may have time to put forward its recommendation before MPs vote, any change in Labour's official policy would have to wait for approval at the party's conference in September - where it can be expected to face stiff opposition from unions as well as a number of senior MPs.