Scrapping Trident would be one of the SNP's key demands to gain its support in the event of a minority Labour government, says Nicola Sturgeon.
The SNP is willing to support a Labour government if no party wins an overall majority - but the SNP leader has ruled out a formal coalition.
Ms Sturgeon also wants Labour to stop Brexit and commit to an independence referendum next year.
The Labour manifesto includes a pledge to renew the Trident nuclear programme.
Ms Sturgeon was asked by Sky's Sophie Ridge if scrapping Trident would be a red line for the SNP to support Labour. She replied "Yes", adding that the SNP would be "absolutely firm" on that.
The SNP leader continued: "I have a moral objection to weapons of mass destruction... I wouldn't be prepared to press a nuclear button that would kill potentially millions, tens of millions, of people.
"But there's also the opportunity costs of Trident - the billions, tens of billions, that are required to renew Trident in my view are better spent on stronger, conventional defence that is more effective to protect our country but also hospitals and schools and better social security provision.
"And these are the choices that we should be thinking very carefully about."
Ms Sturgeon said that in the event of a hung parliament, where no party had an overall majority and the SNP held the balance of power, Scotland would have "maximum influence", adding: "That would be a pretty good outcome I think in terms of making sure Scotland's voice is heard."
She reiterated that there would be no coalition with Labour, saying instead SNP support would be "less formal" - such as a confidence and supply arrangement, where the SNP would support a Labour government on explicit votes in return for government support of specific policies.
"Other matters" that she would want to progress would include holding an independence referendum next year, stopping Brexit, devolving control of migration, employment and drugs classification laws to Holyrood and a "real end to austerity" and "to the misery of Universal Credit and welfare cuts".
She insisted these issues would "resonate strongly with many people across the UK", as well as her supporters in Scotland.
She added that she would "never, ever" put Boris Johnson into power.
Labour say they will not agree to a Scottish independence referendum in the "early years" of government.
And during Friday's Question Time leaders' special, Jeremy Corbyn said he did not plan to rely on other parties for support after the election.
He said: "We're not doing any deals with any other parties. I'm not trying to form a coalition government.
"I'm fighting this election to win it for Labour."
What is Trident?
Since 1969, according to government documents, a British submarine carrying nuclear weapons has always been on patrol, gliding silently beneath the waves, somewhere in the world's oceans.
The logic is to deter a nuclear attack on the UK because, even if the nation's conventional defence capabilities were destroyed, the silent submarine would still be able to launch a catastrophic retaliatory strike on the aggressor - a concept known as mutually assured destruction.
The UK has four Vanguard-class submarines, which each carry Trident missiles. While not on patrol, the submarines are located at Her Majesty's Naval Base Clyde - commonly known as Faslane.
Faslane was chosen to host the UK's Polaris nuclear-armed submarine fleet at the height of the Cold War in the 1960s because of its relatively secluded position next to the deep waters of the Gare Loch and Firth of Clyde on the west coast of Scotland.
Although Labour leader Jeremy Corbyn has been a longstanding critic of nuclear weapons, his party's manifesto for the 12 December election did include a pledge to renew the Trident nuclear programme and spend at least 2% of GDP on defence.
The Liberal Democrat leader Jo Swinson said on Tuesday in an interview with ITV that she would be prepared to press the nuclear button if she was prime minister.
A spokesman for the Conservatives, who launched their manifesto on Sunday, said: "Trident is good for Britain's security, and good for Scottish jobs. Jeremy Corbyn and Nicola Sturgeon want to do a deal that would wreck both."
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