Jeremy Corbyn is under pressure to say whether he would be prepared to use the UK's Trident nuclear deterrent, if he becomes prime minister following the election on December 12.
The Labour leader, a lifelong opponent of nuclear weapons, has in the past said he would not authorise a nuclear strike - even though the party is committed to retaining the Scotland-based nuclear deterrent.
Campaigners have fought for decades for the removal of missile submarines carrying nuclear weapons from their home base on the Clyde at Faslane.
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However, in a round of broadcast interviews, shadow foreign secretary Emily Thornberry repeatedly refused say whether there were any circumstances in which he could order its use.
"The use of a nuclear weapon is a decision on a level that no politician anywhere has to make. It is completely out on its own," she told ITV's Good Morning Britain.
"No-one in the end knows how they would use it, whether they would use it, because it is, it has, such extraordinary force and millions of people can be killed.
"If we are in circumstances where we are under threat, it's impossible I think for any human to say whether they would be prepared to kill millions."
Ms Thornberry said that until recently political leaders, including Margaret Thatcher, had deliberately avoided saying whether they would actually use the deterrent.
"No leader has said one way or the other until very recently whether they were prepared to use the nuclear weapon or not," she said.
"So we've had Conservative leaders recently saying they would press the button, we've had Labour people recently saying they wouldn't press the button.
"I'm of the view that it's best for us not to say one way or the other whether we would use it or not, just as we have done for generations."
Pressed on what Mr Corbyn would do if an enemy had carried out a nuclear strike on the north of England and was planning another on London, she said: "Who knows? That's kind of the point."
For the Tories, defence minister Johnny Mercer said: "If Jeremy Corbyn is unable to make crucial decisions to keep our country safe, he is not fit to be prime minister.
"It is important that Labour urgently clarifies their position on whether or not they would actually be prepared to use our nuclear deterrent if needed.
"Labour are dithering on whether they would use Trident, just like they are dithering and delaying on Brexit."
Meanwhile Mr Corbyn has said a Labour government would ensure British troops were never again deployed in "unnecessary or unjust" military conflict.
In a video to mark Armistice Day, he said: "We must make sure that we never again send our soldiers into unnecessary or unjust wars."