Jeremy Corbyn's close ally Ken Livingstone has been put in joint charge of a review to help decide Labour's position on Trident.
The former London mayor, a long-time opponent of the nuclear deterrent, is to co-convene the party's policy commission on defence issues alongside shadow defence secretary Maria Eagle.
The move is likely to cause fresh anger among Labour MPs, who have already savaged Mr Corbyn over his interventions in recent days.
The Opposition leader has indicated he would not have authorised the drone strike on Islamic State (IS) killer Mohammed Emwazi, appeared to reject the idea that police should have a "shoot to kill" policy against terrorists on the streets of London, and refused to criticise the Stop the War organisation after it blamed the Paris attacks on Western interventionism.
Responding to news of the appointment, backbencher Wes Streeting took to Twitter, linking his post to a report of Mr Livingstone arguing that "the endless interventions of Britain and America and France in Arab countries ... has come back to haunt us".
"Is this the same Ken Livingstone co-chairing Labour's defence review?" the Ilford North MP asked.
It is understood the party's commissions on major policy areas are generally "co-convened" by a member of the NEC and the holder of the shadow cabinet brief.
Recommendations are put to the National Policy Forum (NPF) before being taken to party conference.
Ms Eagle - who supports renewal of Trident - is still believed to be in charge of a shadow Strategic Defence and Security Review (SDSR).
Mr Corbyn, a vice-president of CND, has admitted that large numbers of Labour MPs favour the like-for-like replacement of Trident. But he has said he hopes to win them over to his view before a key Commons vote over the coming months.
Mr Livingstone, who revealed his new role at a book club, risked further inflaming tensions by backing Mr Corbyn's insistence on imposing a party whip for an expected vote on extending RAF air strikes from Iraq to Syria.
Responding to a call from former shadow minister Emma Reynolds for a free vote to recognise "divergent views", Mr Livingstone tweeted: "If pro-war MPs want to support a war they should accept that there is a whip and decide whether to break it."
Meanwhile, a Survation poll found that just 15% of the public thought the UK should carry out immediateair strikes on Syria in retaliation for the Paris atrocity.
Some 52% said Britain should "engage with all countries to co-ordinate an appropriate response" backed by a United Nations resolution.