Nicola Sturgeon says scrapping Trident would be a red line for her party to support a Labour government in the event of a hung Parliament.
She reiterated that she would not go into coalition with Labour, but would enter into a confidence and supply arrangement if they met her conditions.
The SNP leader added she wants Labour to commit to an independence referendum next year and to stop Brexit.
The Labour manifesto includes a pledge to renew Trident.
Ms Sturgeon was asked by Sky’s Sophie Ridge if scrapping the Trident nuclear deterrent 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.”
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Ms Sturgeon said that in the event of a hung parliament, with the SNP holding the balance of power, Scotland would have “maximum influence”.
She reiterated that there would be no formal coalition with Labour, and said “other matters” that she would want to progress would include stopping Brexit, devolving control of migration and employment to Holyrood and ending austerity.
She added that she would “never ever” put Boris Johnson into power.
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 submarines carry up to eight Trident missiles; each can be fitted with a number of warheads.
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 deterrent and spend at least 2% of GDP on defence.
The Liberal Democrat leader Jo Swinson said on Tuesday that she would be prepared to press the nuclear button if she was prime minister.
The Conservatives will launch their manifesto later. Their position is to retain Trident.