Labour's split over the renewal of Trident may be impossible to reconcile, Andy Burnham admitted ahead of a crunch discussion among Jeremy Corbyn's shadow cabinet.
The shadow home secretary said scrapping the nuclear deterrent would be "a step on our own into the unknown" which the UK could not afford in an uncertain world, and called for an "accommodation" to end the distraction.
Labour leader Mr Corbyn is braced for a potentially fiery showdown with his top team when it debates the party's stance on the issue for the first time when it meets later.
Shadow defence secretary Emily Thornberry gave the group a brief outline of her thinking last week, but substantive debate on the party's approach to the looming Commons vote was delayed.
Ms Thornberry, who like Mr Corbyn favours unilateral disarmament, was branded "waffly and incoherent" by one critic after she spoke to MPs and peers about the issue at a meeting on Monday evening.
She told the Parliamentary Labour Party (PLP) meeting she wanted to carry out the ongoing review of Labour policy on Trident in an "atmosphere of mutual trust and respect".
But in the face of hostile questions from MPs and peers, she was forced to tell them there was "no point trying to shout me down".
Mr Burnham, who is a supporter of Trident renewal, said the meeting "confirmed something many of us had long suspected: that the debate on Trident in the Labour Party is going to be very difficult".
He told BBC Radio 4's Today: "There are two positions here which are difficult to reconcile - maybe impossible to reconcile - and the party has got to find some way of accommodating those positions and move forward and don't let this issue take over everything."
In an apparent dig at Mr Corbyn's suggestion that a new fleet of nuclear submarines could be deployed without warheads, he said: "There are some hybrid options that have been put forward but I think most people have found that they just don't work.
"So the discussion has been in the party: 'can you realistically try to find a halfway house?' and most people have concluded that you can't.
"Therefore if there are two positions that are deeply held on both sides but can't easily be reconciled, the party needs to find some way of accommodating that and allowing people to move forward and actually move on to other issues and hold the Government to account."
Interventions at the PLP were "evenly" split three ways between speakers who wanted to renew Trident, those who did not think it was a "binary choice", and those who opposed renewal, according to a senior Labour source.
When former Labour leader Lord Kinnock was asked for his verdict on Ms Thornberry's performance, he replied "Yeah", then walked away, laughing.
Former shadow defence minister Kevan Jones said she had taken questions "but didn't answer any", adding she was "waffly and incoherent".
Mr Corbyn was not present as he was manning a phone bank for Labour's London mayoral candidate Sadiq Khan, but he is expected to attend the next PLP meeting in two weeks' time.
Ms Thornberry played down the criticism, and said colleagues should not be "afraid" of a debate that looked at the options "before people jump to conclusions".
She told Today: "Four, five, perhaps six people at the PLP last night kicked off, but I don't think they necessarily represent the whole of the Parliamentary Labour Party.
"If people proceed with goodwill and if we are prepared to go into this and look at the evidence, the evidence will draw us to certain conclusions. We need to have all the options on the table and we need to have a proper debate within the party.
"We should not be afraid of having debates in the Labour Party and I think this is a debate whose time has come. I think it is a debate that the country needs to have."
Asked if Labour could go into the general election with candidates standing on both pro- and anti-Trident manifestos, Ms Thornberry said: "It is important that we have the discussion first before people jump to conclusions."
She said she maybe "shouldn't have been drawn" on the detailed argument against Trident at the PLP, where she was criticised for drawing a parallel with the need to replace Spitfire fighter aircraft.
Her case - that the development of submarine-hunting drones could mean the sea was "no longer opaque" and render the deterrent ineffective - has been dismissed by former head of the Navy Lord West, a Labour peer who attended the PLP.
He was said by Today to have called the programme to say the claim was "nonsense" because the US and China are spending huge sums ensuring subs remain invisible.
But Ms Thornberry said she had met with "one of the Young Turks" developing the drone technology and was convinced it could be a key factor in the debate.
"I had been challenged and told that it was either Trident or nothing, that it was a binary option, and our view is that it's not necessarily a binary option," she said.
"In order for them to work at all you have to be able to say, with confidence, 'I have a big stick, my stick is larger than your stick' and both sides need to be confident that actually that threat is a real one.
"If we are being left behind by technology, if there is a possibility of cyber attack, if there is the development of drones, then actually these nuclear deterrents may not in fact be nuclear deterrents and that is the issue we need to debate."
She said there were "more subtleties" than reported to Mr Corbyn's suggestion that submarines could be retained but not carry warheads.
And she accused the Conservatives of "playing games" by holding an unnecessary vote on the issue.
"We are having a legitimate review in the Labour Party about this very important issue and if they are trying to take advantage of that, that does not show them in a good light. National security is an important issue and it shouldn't be played with."
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