Sir Keir Starmer has said he would be prepared to deploy nuclear weapons if needed to defend the UK.
The Labour leader said the nuclear deterrent programme is "vital" to the UK's defence so "of course" he would be prepared to use it.
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He would not go into detail about what circumstances would require him to take that sort of action if he became prime minister.
But his comments show his determination to demonstrate Labour as a "changed party" from when it was led by Jeremy Corbyn - a life-long opponent of nuclear weapons.
Asked if he would authorise the firing of nuclear weapons if it was necessary, Sir Keir said: "On the nuclear deterrent, it is fundamental, it is a vital part of our defence and of course that means we have to be prepared to use it.
"Now nobody who aspires to be prime minister would set out the circumstances in which it would be used. That would be irresponsible.
"But it is there as a part, a vital part, of our defence. So of course we would have to be prepared to use it."
Pressed on the circumstances of when the deterrent may be used, Sir Keir said "that is a matter of high confidentiality".
He also insisted his whole shadow cabinet is behind him when challenged over previous opposition from some of his front bench.
In media questions at a campaign event at the Fusilier Museum in Bury, it was put to Sir Keir that his deputy Angela Rayner and shadow foreign secretary David Lammy voted against maintaining the UK's nuclear weapons eight years ago.
Sir Keir said: "I lead this party, I've changed this party... and I've got my whole shadow cabinet behind me.
"I know full well that national security, defence of our country is the number one issue. That is why I voted to renew the deterrent."
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Tap hereSir Keir was taking questions after giving a speech in which he pledged that Labour would be "the party of national security" if it gets into government.
He said Labour will maintain the UK's at-sea deterrent, fund all future upgrades, and oversee the construction of four new nuclear submarines in Barrow-in-Furness.
He also recommitted to spending up to 2.5% of GDP on defence "as soon as possible" - but did not say when that will be.
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The Tories have also pledged to spend this amount, and said they will reach the target by 2030.
Defence Secretary Grant Shapps said Sir Keir's speech was "empty" and criticised him for failing to commit to their timeline on defence spending.
He said: "Starmer's choice for foreign secretary, David Lammy, has described the UK's nuclear deterrent as 'senseless'. This rubbishes the claim that Labour have changed.
"It's clear Starmer lacks the courage and conviction to stand up for Britain's security."
Defence has been a sticky subject for Labour in the past.
Many on the left of the party oppose maintaining the UK's nuclear weapons programme, with dozens of MPs voting against the deterrent system back in 2016.
Mr Corbyn, who was the party's leader at the time, previously said nuclear weapons "didn't do the USA much good on 9/11" and he "would not press" the nuclear button.
Following Sir Keir's speech he said on X: "Nuclear weapons are a profound and existential threat to humanity.
"Instead of investing in weapons of mass destruction, we should be investing in our schools, hospitals and housing to ensure everyone can lead a happy and healthy life.
"That is what real security means."
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The comments were also condemned by the SNP, who called the nuclear deterrent "morally abhorrent".
Shadow defence spokesperson Martin Docherty-Hughes said: "In the middle of a cost of living crisis, it is objectively wrong that Keir Starmer would funnel billions of pounds of public money into keeping weapons of mass destruction on our doorstep in Scotland.
"As tensions across the world increase, Scotland wants no part in the UK government's plans for nuclear weapons which are not only morally abhorrent, but financially reckless."
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