The SNP will "absolutely not vote" for the renewal of Trident, or anything that facilitates its renewal, Nicola Sturgeon has said.
But the First Minister stopped short of confirming that the party's MPs would vote against a Queen's Speech at Westminster which contained proposals on the nuclear deterrent.
Ms Sturgeon was speaking during an election campaign visit to a nursery in Loanhead near Edinburgh.
She was quizzed on her stance on Trident following last night's television debate.
She said: "The SNP has made very clear that Trident is a fundamental issue, so we would never be in any deal with a Labour government who is going to renew Trident and we would never vote for the renewal of Trident or for anything that facilitated that renewal.
"It is one example of where we want to build progressive alliances in the House of Commons."
Asked about whether Trident was still a red-ine issue in any post-election deal with Labour, she said: "If we are in a vote-by-vote basis, then we will vote for things we agree with, but we will absolutely not vote for the renewal of Trident.
"If you had a confidence and supply arrangement and if we are in a situation where there is no formal arrangement, then ... we will absolutely not vote for Trident."
Asked if the SNP would vote against a Queen's Speech by a UK government containing proposals for Trident, Ms Sturgeon dismissed attempts to "get ahead of the election process", adding: "I am making it very clear that the SNP will not vote for the renewal of Trident."
On the prospect of a second independence referendum, she said: "What I have set out is the triple lock that exists on independence.
"Firstly, there would have to be a change of circumstances from the referendum last year for a proposal for another referendum to be in an SNP manifesto in future.
"Secondly, people would then have to vote for that manifesto ... thirdly, people would then have to vote for independence.
"The point I am making is that this is a decision that lies in the hands of the people."
Scottish Conservative leader Ruth Davidson said Ed Miliband has no intention of abandoning his route to power on the coat-tails of the SNP.
The Labour leader has denied Tory claims that he is "ready to barter away our nuclear deterrent in a backroom deal with the SNP" after Ms Sturgeon confirmed her party would vote against nuclear weapons in the Commons.
Ms Davidson visited a farm in Stonehaven, Aberdeenshire, today to promote the Conservatives' rural policies, including a rural business tenancy to lift restrictions on terms and conditions of farm tenancy agreements.
She said: "It looks like the Labour Party's only possible route to Downing Street will be on the coat-tails of the SNP.
"Nicola Sturgeon has made it very clear that a replacement of Trident would be a red line.
"They can't have it both ways by thinking they can cosy up with a strong SNP, and also say they won't take on any concessions that the SNP want to wring from them.
"I don't believe that the Labour Party has any intention of giving up their deal with the SNP if it's the only thing that will put Ed Miliband into Downing Street."
Ms Davidson also said there was no difference of opinion between her and the Prime Minister on the process of another independence referendum.
Last night, she said cannot envisage a situation where the Tories would block another referendum just hours after Mr Cameron said the issue "is settled".
"I don't think that there is anything between us," she said.
"He was asked a question about a second referendum and he said 'it's settled', in that we had a big, bold discussion about the future constitution of our country, we voted in huge numbers and we voted by a clear margin to stay part of the UK and I think we need to build on that referendum result.
"The SNP seem to by trying to ride two horses, saying to people, 'It's okay, we're a safe vote, we're not going to push for another referendum', and then saying to their tens of thousands of new members 'don't worry another referendum is just around the corner and we can push for it in one more heave'.
"Nicola Sturgeon is all over the place on whether there is going to be a second referendum.
"She says that there has got to be a material change and then says an opinion poll may be that change.
"I think a lot of people would have been quite disturbed last night to hear Nicola Sturgeon talking about full fiscal autonomy within the next year."
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules hereComments are closed on this article