British voters will not back unilateral nuclear disarmament, former Labour leader Lord Kinnock has warned Jeremy Corbyn.
The ex-leader's intervention came after the Scottish Labour conference endorsed a motion to scrap the Trident nuclear weapons system.
Lord Kinnock insisted that Labour was getting behind Mr Corbyn, but his comments came as a former Cabinet minister warned the Opposition's economics policies would result in failure.
The debate over the nuclear deterrent has exposed deep divisions within Labour ranks, with Mr Corbyn's support for unilateral disarmament at odds with many members of his shadow cabinet who believe the UK should retain its deterrent.
The Scottish Labour conference in Perth overwhelmingly voted against renewing the Trident nuclear weapon system, putting policy north of the border at odds with the UK party's official position.
The vote also means UK leader Mr Corbyn and Scottish Labour leader Kezia Dugdale - who supports a multilateral approach - have different views from each other and from their own parties.
Lord Kinnock said: "The debate is wide open. What I do know is the British people will not vote for unilateral disarmament. And that reality has to be dealt with."
The latest show of unease within Labour ranks at the policies being adopted by the leadership came from former Treasury minister Liam Byrne, who warned against pursuing policies of renationalisation and "spiralling public spending".
He also hit out at the idea of "printing money" through the so-called "people's quantitative easing" backed by Mr Corbyn during his leadership campaign.
But Lord Kinnock insisted Mr Corbyn and shadow chancellor John McDonnell were right to set out an alternative economic approach to George Osborne.
He told the Press Association: "It's clear that the falsehood of George Osborne's economics has to be exposed and the alternative for investment-led recovery properly presented. That's the duty of the Labour Party now.
"And everybody's got to get on with it."
Lord Kinnock defended Mr Corbyn's approach to Prime Minister's Questions, where he has taken to asking David Cameron questions submitted by members of the public.
Asked how he thought Mr Corbyn was performing in the weekly Commons showdowns he said: "Fine, it's authentic and that's the important thing. This isn't some kind of show being put on.
"That's Jeremy. Which is the way it works. It's the genuine article. About everything else, time will tell."
Mr Byrne said Mr Corbyn's leadership election victory "confirmed the moderates lost the argument" and "the New Labour-Millbank era of politics is over".
He was sceptical about the economic policies championed by Mr Corbyn and Mr McDonnell.
"In his leadership campaign, Mr Corbyn proposed many things on which we agree - tax justice, rebalancing the fiscal burden, welfare efficiency, industrial policy, a green plan and devolution.
"But I worry that People's QE - printing money - renationalising things and spiralling public spending risks failure.
"But if Labour's moderates don't think this approach will work, then we need to show a better way that will - a better way to challenge the inequality we all hate."
Mr Byrne told the Policy Exchange think-tank Labour should be "the pro-enterprise party in Britain", but his comments came as the CBI expressed disappointment that Mr Corbyn would not be addressing its annual conference.
The CBI said it was told the Labour leader was too busy to attend the event in London next Monday.
Tony Blair's former special adviser, Alastair Campbell, was scathing about the decision not to address the CBI, suggesting it would help clear the way for George Osborne to succeed David Cameron as prime minister.
He tweeted: "If @jeremycorbyn is seriously saying he won't put Labour's case to the @CBItweets we may as well just give Osborne the job now."
Meanwhile, former minister Tom Harris, who lost his Glasgow South seat as Scotland's Labour MPs were swept away by the SNP landslide, hit out at the current state of the party.
Shadow foreign office minister Catherine West reportedly suggested the party would consult the Stop the War Coalition - of which Mr Corbyn is a former chairman - over any proposal for UK air strikes in Syria.
She later insisted her comments had not been addressed to the campaign group but to a group of Syrian refugees at the meeting, but a Labour spokesman confirmed the party would "listen to representations from the Stop the War Coalition, as it would from other external bodies, before coming to any decision".
Mr Harris wrote on his Facebook page: "After 60% votes for sure-fire election losers, IRA-supporting shadow chancellors and Scottish Labour unnecessarily splitting the party on issues over which it has no responsibility, we have a shadow minister telling Stop the War - a madcap coalition of Trots, Islamists and anti-West fury chimps - that Labour will consult them on how it will vote on Syria.
"So that's it. Labour has jumped the shark. It has gone from 'a bit bonkers' to 'irredeemable' in the space of a single day.
"And I give up. That's it for me. Giving. Up."