Jeremy Corbyn has said he would not push the nuclear button if he was prime minister.
The Labour leader said he did "not believe" Britain should be spending £100 billion on renewing Trident.
Asked if there were no circumstances in which he would use the country's nuclear deterrent, he said: "There are five declared nuclear weapons states in the world, there are three others that have nuclear weapons, that is eight countries out of 192."
Pressed again on BBC Radio 4's Today programme on whether he would use it, he replied: "No. 187 countries don't feel the need to have a nuclear weapon to protect their security, why should those five need it themselves?
"We are not in the era of the Cold War any more, it happened, it finished a long time ago."
He added: "There are many in the military who don't want Trident renewed because they see it as an obsolete thing that they don't need, they would much rather see spending on conventional weapons and conventional forces and I would like to see that money also used to invest in manufacturing industry and protect those very high skill jobs that for the moment are used making the weaponry and the vessels that go with them."
Mr Corbyn said the party would have a review of defence policy as a whole and Trident would be included in that.
He said: "My view is very very clear, I put that very clearly out there during my election campaign for the leader."
He added: "I am opposed to the use of nuclear weapons, I'm opposed to the holding of nuclear weapons, I want to see a nuclear free world, I believe it is possible." Mr Corbyn said he was "well aware" of the different views present around the shadow cabinet table.
He added he was also "very well aware" of the views of a large number of party members who were "quite prepared to vote for me knowing full well what my position was on nuclear arms".
He said: "It would be dishonest of me to say anything less than my honest view on it."
Mr Corbyn made clear in his speech to conference that he believed his overwhelming victory in the Labour leadership contest gave him a "mandate" for his position on Trident.
And he left no doubt he would scrap the system, saying: "I don't believe that £100 billion spent on a new generation of nuclear weapons taking up a quarter of our defence budget is the right way forward.
"I believe Britain should honour our obligations under the non-proliferation treaty and lead in making progress on international nuclear disarmament."
Mr Corbyn said the policy on Trident would be developed through a review, but the reference to the scale of his victory in the leadership contest will be seen as a sign that he thinks the members and supporters who voted for him support his position.
His stance was greeted with applause in the hall, but reporters watching the speech in Brighton noted that the majority of members of the shadow cabinet did not join in as the leader issued his defiant comments about Trident.
Asked whether his mandate from Labour members meant he was ready to overrule MPs on Trident, Mr Corbyn told the Press Association: "I was being completely honest about it. I fought the leadership election campaign and included in the 10 points I put forward my opposition to renewal of Trident and my desire to see a nuclear-free world and asked Britain to fulfil our obligations under the Nuclear Non-Proliferation Treaty.
"It would be less than honest of me to say anything else yesterday, because that clearly gave me a mandate. I got 60% of the vote - 250,000 people voted for me knowing full well what was in my policy statement. Every one of them received a copy of that policy statement.
"There are people in the Labour Party - mainly Labour MPs - who support the renewal of Trident, partly on the jobs argument, partly on defence arguments.
"On the jobs argument, I also made it clear that I didn't see the loss of jobs as an option in this. I saw protection of those jobs and investment in high-quality engineering jobs as the way forward.
"In fact the cancellation of Trident would give us an opportunity to improve the manufacturing base of our society.
"There's also quite a big defence argument surrounding Trident, where you will find many people in the military who don't want a quarter of the defence budget taken up by Trident, don't see Trident as something they would ever want to use and think that in the modern world the levels of security Trident offers us are not appropriate when the real issues are other forms of defence against threats."
Asked whether he would personally be ready to use Britain's nuclear weapons as prime minister, Mr Corbyn told PA: "I would want a Labour Government to do everything it could to bring about the possibility and the option of a nuclear-free world. I've attended nuclear non-proliferation conferences in New York and Vienna and Geneva, and I'm very committed to the principle of a nuclear-free world.
"Britain has played quite a considerable part in that, but I think we need to go a bit further."