Shadow Foreign Secretary Emily Thornberry has told ITV News the Labour leader will do "everything it takes to protect our country," if faced with conflict, but refused to say if Jeremy Corbyn would use nuclear weapons in such an eventuality.
A lifelong opponent of nuclear weapons, Mr Corbyn has in the past said he would not authorise a nuclear strike - even though the party is committed to retaining Trident.
Defending her party's leader, the shadow foreign secretary said: "Nobody should be 'okay' pushing a nuclear button where millions of people will be killed as a result of your actions."
Referencing the views of a former Conservative leader from the Cold War era, she said: "Margaret Thatcher was on record as saying 'I am not prepared to say. We have a deterrent, I may or may not use it. I'm not going to go through the circumstances in which I might'
"A deterrent has to work in that way. That very ambiguity is part of what defence is."
She earlier outlined the same stance to ITV's Good Morning Britain: "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."
"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."
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."