Liberal Democrats voted down a campaign against Trident after the party’s Portsmouth leader joined calls to maintain the nuclear weapons.
Tim Farron won his first crunch vote as Lib Dem leader after the party rejected a proposal to scrap the Royal Navy warheads.
The Lib Dem leader had pleaded with grassroots activists not to back a policy motion committing the party to oppose plans to decommission existing warheads at the earliest possible opportunity.
Following a concerted effort by the Lib Dem establishment an amendment backed by the leadership, committing the party’s MPs and peers to vote against the like-for-like replacement of the Trident system but not endorsing unilateral disarmament, was passed by 579 votes to 447.
Mr Farron, who was in the conference hall in Bournemouth for the vote, said he was “very pleased with the result”.
Gerald Vernon-Jackson, who stood in the Portsmouth South seat at the general election, warned activists that passing the motion in favour of unilateral disarmament would be “like the Labour Party voting for Jeremy Corbyn - it feels nice at the time but it will lead to electoral disaster”.
It would be a “hammer blow” in Tory-facing seats and could “torpedo” Mr Farron’s leadership, he said.
Former MP Tessa Munt said the motion gave the Lib Dems a “completely clear position” on nuclear weapons.
She questioned the need for Trident, telling activists “you cannot nuke a terrorist”, and also questioned the expense of replacing the system in a time of austerity.
“How can this country have food banks when we are just about to spend £100 billion on a replacement system that we will never use?” she told activists in Bournemouth.
Julian Huppert, another of the party’s MPs voted out in May, said: “Our best chance for nuclear disarmament is to show that we will take action ourselves.”
But party grandees including Baroness Williams, Scottish leader Willie Rennie, Welsh leader Kirsty Williams and former deputy leader Sir Simon Hughes lined up in support of the amendment, which will establish a working group “to develop policy on the future of Britain’s nuclear deterrent, if any, following a full consultation within the party”.
Mr Rennie also made clear a defeat for Mr Farron would deliver a blow to his leadership.
“He has chosen to lead on this issue,” Mr Rennie said. “I don’t expect you to blindly follow everything the leadership says, but I think there are sound reasons why on this occasion you should back Tim and his proposals.”
After winning the vote Mr Farron said: “I’m very pleased with the result. We have restated our commitment to vote against the like-for-like replacement of Trident and we have maintained a multilateralist policy.
“We believe that we should disarm but we should disarm internationally alongside other countries, with Britain being at the table arguing the case for disarmament.”