Jeremy Corbyn has faced criticism from senior Labour colleagues for saying he would not use Britain’s nuclear weapons if he were prime minister.
The long-time opponent of nuclear weapons told the BBC he was “totally and morally opposed” to their use, saying the Cold War era is over.
Asked if he would ever press the nuclear button, Mr Corbyn said: “No.”
Shadow defence secretary Maria Eagle said the words were “not helpful”.
“I am opposed to the use of nuclear weapons. I am opposed to the holding of nuclear weapons. I want to see a nuclear-free world. I believe it is possible,” Mr Corbyn told BBC Radio 4′s Today programme.
Shadow chancellor John McDonnell backed Mr Corbyn’s position, saying it was “a moral issue”, but shadow justice secretary Lord Falconer said the leader would have to abide by the party if it decided to retain Trident.
During his Today programme interview, the Labour leader also said he believed he could be prime minister.
He defended the lack of coverage of the deficit in his speech, saying he was “setting out some general philosophical ideas” rather than policy details, and he said immigration was “not a problem, but a great opportunity”.
In his conference address on Tuesday, Mr Corbyn said his landslide leadership win gave a “mandate” for his views on disarmament of Britain’s nuclear weapons.
But a large number of his shadow cabinet, including Ms Eagle, and many Labour MPs do not support his position.
Labour is to hold a review of defence policy, which will include future nuclear capability.
Analysis, by BBC political correspondent Ross Hawkins
It did not take a debate, within Labour or the House of Commons. A few words on the Today programme did the trick.
Should he get to Number 10, he said simply, he would not press the nuclear button.
Think of it this way: Corbyn declared to Britain’s potential enemies that with him in charge they could disregard a multi-billion pound weapon system.
Or, perhaps, put it like this: a man with a lifetime commitment to scrapping Britain’s deterrent promised not to kill untold thousands of innocent people if he had the opportunity.
Many politicians choose not to be so frank.
Mr Corbyn told Today: “I do not think we should be renewing Trident. I think we should be fulfilling our obligations under the Nuclear Non-Proliferation Treaty.
“I think we should be promoting an international nuclear weapons convention which would lead to a nuclear-free world.”
Spending “£100bn” on replacing Trident was misguided, he said, arguing that the money could be spent on more “conventional” equipment and forces, and protecting “high-skilled jobs” of Trident workers.
The Labour leader said there were five declared nuclear weapon states in the world and three others that had nuclear weapons.
He added that 187 countries “don’t feel the need to have a nuclear weapon to protect their security”, and asked “why should those five need it themselves?”.
When pushed by Today presenter Sarah Montague on whether there would be any circumstances he would use the nuclear option, he said: “No.”
In a later interview with BBC political editor Laura Kuenssberg, the Labour leader responded angrily to the suggestion he would put his principles before national security.
“We are not under any threat from any nuclear power. We are not under threat from that. We are under threat from instability,” he said.
Mr Corbyn said he was aware of the different views in his shadow cabinet, but also pointed out his supporters “were prepared to vote for me knowing full well what my position was on nuclear arms”.
And he has rejected the idea that the defence review was pointless given his position, saying “nuclear weapons are not the only issue in defence”.
However, shadow defence secretary Maria Eagle told the BBC “I’m surprised he answered the question in the way that he did”, saying it “undermined to some degree” Labour’s policy process.
Labour’s current policy was in favour of retaining a nuclear deterrent, she said, adding: “I don’t think that a potential prime minister answering a question like that, in the way in which he did, is helpful.”
Mr Corbyn’s comments were endorsed by his shadow chancellor, Mr McDonnell who told the BBC: “I don’t believe the use of nuclear weapons is morally or legally appropriate, therefore we have to accept that.”
He said the Labour was “just saying what he means and I agree with that”.
Trident is the UK’s sea-based nuclear weapons system – made up of submarines, missiles and warheads. A decision on whether to renew Trident is due to be taken in 2016.